I know how you feel man, just getting back into the competitive game myself, and I haven't been doing to hot as of late. What needed to be said as pretty much already been mentioned, but I'll give my two cents just for the heck of it.
Obtain the most EV's & IV's and use them in the most beneficial way you see fit. Expect to put a lot of time into training if you are looking to get any better. Tons of players on this site, as well as online more than likely have maxed out their EV's in their teams, and probably have decent IV's as well. You gotta know that is what you're up against, so train your team to be on par.
Construct your team based off of a specific "theme", if you will. e.g, Baton Pass, Offensive, Defensive, Annoying, Hazard, Turtleish (lol), etc. I can tell Smogon.com has guides on pretty much every pokemon, from Overused to Neverused, for ways in which you can play these specific Pokemon in battles, as well as know their capable roles.
Prediction, as mentioned somewhere above, is also important. You have a short amount of time, to think, and take in a lot of consideration regarding yours and your opponents next move in a battle, nonetheless, take your time, and think before you make a move, also ask yourself why you are making the move you are about to make. Also, with taking in the consideration of what pokemon your opponent has on the field, think how your opponent might respond to what you have. You can counter by switching out, or using a move he/she perhaps might not have expected.
And if you win the battle, don't forget to mindcrush him/her.