I usually take one plate for the main meal, and then one helping of desert. I load my "main" plate fairly heavily, but not so that there's a mountain on there.

I don't go to buffets often though. There's one in the local area that I used to really enjoy, but the quality of food has deteriorated massively, and now stuff sits there until it's dried out and disgusting I'd rather a small helping of nice stuff than an all you can eat pile of junk.