I stopped going to all-you-can-eat buffets years ago for the simple reason that you don't stop eating when you're full - you stop eating when you hate yourself. If the number of all-you-can-eat buffets drops, it's likely because society is starting to advocate healthier lifestyles in my opinion.
Hell, I struggle with even normal restaurants. I like going out, my wife and I can afford it, but I feel like I'm going out for the freshly, crispy yummy food.. and yet, I always end up taking half of it home because they hand me a massive plate. I'm thinking next time we go out we'll try and get the same thing and split a plate. .. and then tip as if we ordered 2 plates because I feel guilty to the server. Hah.
The server isn't the one making the food so no reason to feel guilty, splitting an entree is normal, but if you really want to help out the restaurant (assuming you do this often and at the same place) order more drinks from the bar or preferably a bottle of wine for the table.
That does more for the entire staff, than just the server who quite frankly earns way more than they provide to the operation.