You need to make sure you are up to date on sets that Pokemon normally have and current threats in the metagame. For example after Generation 4 came out Azelf (learning stealth rock, being fast, having a focus sash and being able to explode) became the most OVERUSED lead.
Therefore within 5 months Machamp leads (with Payback and bullet punch) became the standard lead to face since they countered Azelf extremely well.
Moreover know general moves that Pokemon are likely to have can also give you an edge when it come to making the big prediction that can help you.
For example my Metagross had just KO'd a Pokemon, the next thing I see is a Dusknoir. Obvious Will-O-Wisp is obvious, so I switched to my Swellow so that it activated Guts for me. I then used U-turn to Swampert fearing the thunderpunch (but actually it was ice punch that came my way).
= Wrecked Dusknoir.
You don't have to use SUPER SPECIAL sets since people use the sets on Smogon and Serebii for one reason....they are proven to be good. However mixing it up can lead to confusion and will therefore give you an edge.
i.e. scarfed Flygon is common, but scarfed Dragonite isn't.
Special Mence is to be expected, but not a special Dragonite.
These mind games use the metagame knowledge of the opponent to their disadvantage where they bring in something which can't take a Draco-Meteor or was expected to outspeed and KO.
May sound odd, but using Serebii and Smogon for ideas on what moves to use complimented with Psypokes for the full movepool of the Pokemon in question can give you an idea of Type Synergy, Originality and whether the team looks too physical / special orientated.
Moreover if you don't know about breeding / IV values / EV training / PkRs then Psypokes will help you learn about them in order to bring a good Pokemon to the arena of battle that is specialised in the correct areas for the most effective impact.
An idea of who you really want to use.
All teams are based around a specific type / Pokemon or two Pokemon. If you have an idea of who you want to start with them building the team around them to include Type Synergy / Offensive Synergy and Defensive Synergy (ability to deal with potential Offensive threats and Deffensive walls).
Normally one Pokemon will do, trying to get three Pokemon who you think are "Awesome" might not be wise...check the type synergy first.
Your basic team layout, by this I mean not specific Pokemon but the general roles of your team.
For example I like...
2. Lead Support
I like the two leads because a generic lead (in Gen V = Deoxys Speed) is very obvious and I like to distrub people's rhythm right from the start, i'm more offensive but remember you don't have to be. A generic anti-lead does nicely for me (Examples are Gallade with Shadow Sneak and Zen Headbutt for Azelf and Machamps, Persian obviously and Ampibom).
Whereas my second lead "lead support" is normally used incase I want to set up my rocks or there's a double switch late on in the game and I need a good place to start from. Remember Leads must be designed with the intention to start off "from scratch" e.g scarf / u-turn / fake out is a good starting point but in particular the lead support should be able to deal with the remaining leads that the primary lead can't deal with.
I like to have a main central core of the team, normally using types of Pokemon who are involved in a three way triangle of types therefore they can generally take care of other types that come my way.
My tester and successful core at the moment is..
Vaporeon, Celebi and Zapdos
Firstly Zapdos has general bulk and can also be used in a semi-offensive way with Thunderbolt / Discharge (for the paralysis) when combined with Heatwave or HiddenPower (Ice / Grass). It also dodges the obvious earthquakes and is pretty speedy making it a good semi-wall.
Vaporeon deals with Sub Heatran like a Boss, moreover with it's Wishing it acts as heal support and Toxic support at the same time.
Celebi can not only Leech Seed for increased heal support but it can calm mind up and with Psychic and Thunderbolt / recover you don't have much to worry about (Metagross = Leech Seed, then switch).
A good idea would be to have a mix of status / healers / tanks / offensive Pokes in this area.
Other good Pokes to have here are...
Suicune (offensive, calm mind, subsleeptalk, status. Whatever floats your boat)
Raikou (offensive, sub calm mind etc)
Rotom (Wash / Heat in gen 5)
Basically anything with bulk / good offense so that your set can be offensive or defensive but remaining unpredictable and forcing a switch / doing damage otherwise.
Remember don't get carried away Salamence, Tyranitar and Metagross seem like an awesome threat..but with generic weaknesses that you generally see on more than one Pokemon (like Heatran, earth power, fire blast, hidden power [ice]) Synergy of this core should be immaculate since this should be the bulk of your damage dealing team. In this scenario you have to switch out of the Heatran and into a Lead (not really designed to take hits) or your Cleaner.....this becomes more problematic if you Heatran happens to be a substitute one.
The Cleaner should literally be something that "cleans up" / "sweeps" either late game or for a revenge kill before getting back. Moreover this Pokemon should be the final "Glue" of the team, in my team at the moment I found I didn't have Fighting or Grass on my team....therefore a Nasty Plot Infernape with Vacuum Wave and Grass Knot fills the roles nicely. Add the fact that Nasty Plot Ape isn't common and you have a Pokemon that can induce sleep (Persian) giving a good chance to set up "LATE GAME" can result in a tidy sweep.
Late Game Mence / T-tar / Nite (Dragon Dancer)
Another standard set is :
5. Support / Status healer
4. Being something like Skarmory (access to Roar and Whirlwind). Now in generation 5 with Dragon tail more possibilities have opened up for this.
*the last two are due to perish song phasing.
5. Being something with access to Leech Seed, Wish or Aromatherapy / Heal Bell. Whose main purpose is to support the core and phaser when they are injured in battle.
N.B. only one lead to set up stealth rocks / entry hazard, even though this is normally left to the phaser it means the team becomes extermely vulnerable once the lead is taken out. Since your "core" isn't designed to come in on a double switch / another Poke fainting and get the momentum of your team going back in your favour.
Now you need to think..
"Where does my choice Pokemon sit in this team"
Depending on which section it sits in depends on what part of the team is built up first and which part is build around it, generally though the Cleaner is the LAST one to choose.
A good idea is placing it in the Core of your team, a solid core makes a team very strong while being diverse enough to withstand some punishment. Remember though the synergy must be good, you should be able to say to yourself how your would deal with the top 10 threats to a specific Pokemon (chances are you'll run into two of them per match).
-Protect / Detect - An underestimated move, normally spammed on Vaporeon / Umbreon for perfect Wish healing. However using the move on such Pokemon as Gengar or more fragile Pokemon can give you a nice "scouting" bonus.
e.g. you don't know why a Jirachi was switched into your Gengar...*Protect*. Upon seeing that Jirachi uses Zen Headbutt / Iron Head you would expect that it was scarfed or carried Fire Punch.
Therefore switch to Heatran, either the opponent will be a mug and stay in or they'll double switch.
+ point, you've just learnt a lot about the jirachi without taking any damage
-Weather Teams - A risky strategy, while they are unique and rare in generation 4 since they only consist of Ice and Sandstorm with the later normally just coming about because of T-tar's ability and not correlating to the rest of the team. However even with the type restrictions (to only a few who will take the residual damage) if you make some crazy sets or alter normal ones to your fit (i.e. Clefable with Burn Orb / Trick and Blizzard for sheer Power) then you can catch people off guard by how much you can dish out.
Think more carefully about these though, since the Core tends to be more fragile and prone to certain threats.
In Gen 5 Rain Dance and Sunny Day teams become less "hap hazard" and so become more of a viable option. However pulling off a sunny day team in Gen 4 can really cause distrubtion in the opponents mind.
-Trickroom - drawing parallels with the weather teams, slightly risky making the whole of your team Synergized with "Trickroom" conditions since when Trickroom comes down you become incredibly exposed. The real question is...are you brave?
So you got your choice Pokemon in place, you have an idea of the kind of team you want to build (defensive or offensive). Now it's time to complete the team. This process should take the 2nd Longest out of all the sections, remember consider all avaliable options and relate that to the "resources" and the current Metagame.
N.B. vintage sets and tactics are old school but they aren't always expected!
Battle, fight it out. The only way that you'll come to know if the team is good or not is if you test it out. If you lose the first game BUT feel that it was down to your fault then in my opinion the team is good but you don't know exactly how to use it strategically.
If you're winning / losing on an equal plane then this is the best scenario, we always lose games based on "Crits" or "Hax" or "Speed Ties" meaning that there's always an element of luck when winning, however if you're winning the majority of the games you have with your team....well it shows that..
1. You're knowledgable and able to make good predictions
2. The team is one that has complete synergy
3. You definitely know how to use the team to the best of its strengths.
Learning how to use the team is probably the most important since the metagame knowledge (part 1) should already be covered. Remember while creating your Pokemon you should know what it will be able to take a hit from and what it won't take a hit from. (Knowing the basic EV spread of generic sets and that of your own).
Finally if you're in doubt over a team that you like but want to improve then post a RMT (Rate My Team) up, remembering all the details so that others can contribute.