Once you've learned position and TAG, selelctive agression is definately next.
Now it's time to stop playing cards and start playing poker. Poker is a confidence betting game. You know how to play cards, the cards now become tools secondary to playing people.
Go play the 50 cent and 1 dollar tournaments, never limp, ALLWAYS raise, even if your the BB and got nothing don't check .....RAISE!
You can afford to lose a dollar use these little tournaments to experiment. Don't minimum raise, everytime you come into a pot raise at least 3x BB.
Watch how people react, when you raise too often, they become suspicioous of you, some of them will start calling you with inferior hands. make note of who they are, you'll get a chance to pounce on them later.
Notice who's scared and who's a gambler, who you can pick on, who will think you have the nut or trash, file your impressions of these people in your mind and use that information when you need it.
Your chips are tools of the trade, limping and minimum bets don't get you information and information is what you need to make good decisions.
Now don't go in playing a maniac, that's not the purpose of the excercise. Play TAG in the beginning allwats raising, never limping, develop your table image. The table need to know that if they get in a pot with you it will be costly in the early stages.
As a tournament progress's you can get LAG in the later stages, isolating the opponents you've got information on.
Iv'e done this several times and I still do just to keep my edge, using the 1 dollar tournaments as practice grounds. You'll find that when your hitting your flops and winning the showdowns early it contributes to your table image immensely.
If your not hitting your flops and are getting called down with better hands and being outdrawn on your image drops huge. There comes a time when you've bet too often too much, and the table will no longer respect you.
CHANGE GEARS .. tighten up ...ROCK
If they are scared of you, bully them, if they aren't they need reason to be scared, rock up and slam them with quality.
Sometimes your negative swing for one or another given tournament is insurmountable, other times the cards will be hitting you over the head and your trash speculative hands will make you a table terrorist. Most of the time it's somewhere in between, practicing aggression and reading skills in these little tournaments can give you experiance that adds to your confidence.
And confidence is what aggression needs to win.
Good Luck.