The only tip I can think of (and I hope someone didn't already mention it) has to do with eating at the buffet.
When I eat there, I allow myself to have one plate worth of food. Once that food is gone, no more. Of course, this requires a bit of self control (i.e. a plate full of bacon might not be the best choice, neither would a plate that is overflowing with food), but it seems to be helpful.
RCCL - Adventure OTS (Honeymoon!)
CCL - Carnival Miracle
RCCL - Enchantment OTS