Okay, so I know Annie is a relatively easy champion to play; but every champion has his/her neat little tricks, which can help you to dominate even further.
- Try to avoid running around with your passive active. It's really obvious, and it will only serve to scare your enemies away, robbing you of a potential kill(s). The swirling animation is surely clear to all players by now as an excuse to stay clear and not group up. Instead, build 3 charges. This way, you can activate your E (which is essentially something you pop for a fight as it grants defensive stats/effects anyway) as soon as you're ready for a fight. The activation of your E will trigger your passive on your next offensive spell, and your doing this will disguise the fact that you have a stun ready to all players who don't manually check your buff bar.
- Annie is a farming machine; to top things off, her Q is free when the target is killed. Level your Q primarily, allowing you to farm very effectively, and also granting you a means to nuke your opponent should the opportunity arise. You're a tough champion to fight against, and I don't envy your opponents.
- Tibbers was designed to be a drop-bear. Use the above 'disguised passive' method before dropping Tibbers on an enemy (or multiple enemies). If they don't see the swirling animation, they might not expect it, and you might just get an opportunity to stun/nuke an entire team.
- Give your opponent no choice but to submit to your superiority - if he/she is passive, use your Q to farm minions mercilessly. If they're aggressive, or they were passive until they realised how farmed you're getting? Show them how much damage your Q/W do, and scare them back into being passive so you can farm. Either you'll get champion kills with your minions, or you'll get the minions for free; either way, you'll come off best.
Annie is relatively easy to play, and it's not exactly difficult to be ridiculously powerful. But with the preceding tips, you can make her even more ridiculous, so, why not? :-)