I posted on this topic before:
Read entire thread: http://www.jehovahs-witness.com/topic/109423/do-disassociate-anull-your-baptism?size=20&page=1
If you read the elders hand book you can see that the only way to get your baptism annulled is if you where participating in sinful behavior before during and after your baptism that, if the eldest knew would have stopped you from getting baptized. They are advised to contact the branch to make sure thou, maybe there will be a way to disfellowship anyways...
It is worth a try if you want a "clean" exit.
If anyone want's to try it I would suggest that you first read the "rules" so you can play their "game".
Then you could go to the elders with a story (if it is fabricated or not wont matter). You can tell them that you where watching porn before during and after your baptism, you can say that you are distraught because you know deep down you never really got baptized and you hate living a lie, you never even gave your life to Jehovah in prayer... You just gave in to peer pressure etc.
Then if they decide to "grant your wish" you can start to fade instead of making progress towards your "real" baptism.
It is dishonest I know, but why play by their rules?
I don't know if this works in practice but it think it would have the best chance of working. I don't think bringing up doubt and reasons you don't believe anymore is going to help, that might just reveal the truth, that you are trying to escape without being DFed.
Yeah, if anyone tries please let us know how it goes.
One of the reasons it might fail is that the elders manual makes it very hard to actually pull it of. And if you say all the right things and manage to persuade the BOE, they are then supposed to call bethel for future instructions. So they might not grant it even if the BOE think it needs to be annulled...
It is important that you where sinning before and during the entire process because that is proof that Jehovah could not have accepted your dedication to him.
Don't forget to read everything about this before trying it, the elders manual is a good place to start.