Let's see -- you can go back more than once, so heaping your plate isn't necessary. When you've finished a dish, leave it on your table, go up and grab a clean one, and start over! An attentive waiter should clear the old dish (asking if it's okay first) before you're back.
...take a walk down the buffet line if at all possible to have a general idea of what you're going to get. You don't want to fill up on things that look interesting and then find out you could have gorged on the one thing you can never get enough of. This will also help you pace yourself 1st course vs. 2nd course. In some buffets the soups/salads will be particularly attractive -- in others it's meat and dessert!
If the food is unfamiliar, take a small spoonful of each thing, and go back for more of what you like (best approach at someplace like Boma, where more of the food may be unfamiliar than usual). One of the real attractions of a buffet is guiltless sampling, so it's the best of times to discover whether you've acquired a taste for something you last tried at age 10 (without paying for a full entree of it!)
Don't forget veggies!
Don't mix wet and dry food too much (okay, personal quirk -- don't like wet and dry touching!)
When in doubt, choose the option you can't make at home/get in a local restaurant.
