You cannot, you got banned for one or other reason! Was there a reason mentioned? All you can do is try to appeal, but not much people succeeded that way. Look for alternatives....or register new account on different address, from diffr. IP, diffr. name...everything different.
Once you loose a adsense account its nearly impossible to get that one back again but you could try again using differant details or use something else.
The others are right, I'd go further and say you stand no chance of getting it back. You don't mention your url so I can't comment on what might have got you banned. Dickie
Its very tough to get in adsense once banned. Just try to shift to another network like mediashakers or adbrite
Yup appeal to reclaim your account. Just make sure you collected necessarily resources to prove you did not do anything wrong. Also, the appeal can only be used just a single time. Make a good use of it.
If you are really genuine in your activities, then email Google Adsense team. They will look into it.