I had my lateral incisors (I think that's what they're called) built up; they were TINY. I think the most common ways to build up teeth are through veneers and composite bonding, but there may be others. I elected to go the bonding route because it's MUCH cheaper than getting veneers (I paid around $400 total for both teeth, then got reimbursed a chunk of that from my insurance).
The procedure is pretty easy and painless, and it's done by a dentist. Basically, the bonding material is sculpted to your tooth in the desired shape, then hardened with a curing light. Before the bonding material is applied, the tooth is prepped to roughen its surface, which I guess helps the material bond to the tooth. Usually, anesthetic is not needed.
I had my teeth bonded while I was still in braces (the ortho removed the brackets from the affected teeth right before the procedure, and put them back on right after), but I think a lot of people wait until after their treatment is complete. I'm not sure why my ortho wanted the bonding to be done during treatment.
I really like my bonded teeth, you can't even tell the difference between the buildups and my "real" teeth. I'd recommend shopping for an experienced cosmetic/aesthetic dentist to do the procedure though, apparently it's kind of artform that not everyone can do well.