These rules should help all of you out there with your team building skills. These are basic rules of competitive battling.
1. Do not use two moves of the same type on a Pokemon unless they do completely different things. Something like a Staraptor with Aerial Ace and Brave Bird is never a good idea. Something like a Scarf Typhlosion with Eruption and Overheat is perfectly acceptable.
2. It's usually not a good idea to put both physical and special attacks on the same Pokemon unless you have a very good reason (ie. physical Fighting move on a special-based Poke to take out Blissey). Very few Pokemon are viable as mixed sweepers, and even then they usually only have one physical and three special attacks or vice versa.
3. Do not use moves that take two turns to set up. They give your opponent an advantage and opportunity to switch to a counter. These moves include: Hyper Beam, Giga Impact, Blast Burn, Hydro Cannon, Frenzy Plant, Fly, Dig, Dive, Sky Attack, Skull Bash, and Solarbeam. It is always better to use a move with a lower base power twice in a row then lose a turn.
4. Make sure you know what item you're giving your Pokemon to hold, and avoid Hax Items. Hax Items are items that unfairly increase luck and are frowned upon in competitive play. These include: Focus Band, Quick Claw, and a few others. The most useful items for competitive format are as follows: Leftovers, Life Orb, Choice Band, Choice Specs, Choice Scarf, Focus Sash, Expert Belt, BrightPowder, Shed Shell, Flame/Toxic Orb, Pinch berries (Liechi, Petaya, and Salac), and other useful berries like Lum or Yache.
5. Make sure you give your Pokemon useful natures. Don't be afraid to keep breeding until you have a good nature, IVs, ability, etc. It always helps you in the long run. No one wants to be stuck with a Naughty Blissey or Modest Garchomp.
6. KNOW HOW TO USE EVS! I cannot stress this enough. If you want your team to be rated, it is incredibly helpful to post your Pokemon's EVs, and not just their stats. Nobody has time to memorize max stats for every Pokemon, so it is to everyone's advantage for you to give your EVs as a general idea of how your stats are distributed. If you don't know what EVs are, PLEASE GO HERE:
http://www.smogon.com/rs/articles/evs_ivs OR HERE:
http://www.serebii.net/pokedex-rs/ev.shtml