My most important cooking utensil: High-quality knives!
When preparing the dieter's staple of low-fat chicken breasts, it is much easier to cut them for a saute or fajitas if they are PARTIALLY frozen.
Many dishes taste great if you veer from the recipe and use lots more veggies than the recipe calls for. This gives you more "fullness" for less calories. (i.e. I make cabbage rolls with 1/2 the meat called for, and use peppers, celery, and onion to replace it)
Experiment with recipes. Can you make that recipe that calls for eggs with just the whites and leave the yolks out? Does the recipe still work with 1 tbsp of oil instead of 2?
You won't have to sacrifice flavor in most cheesy recipes if you reduce the cheese amounts by 1/3 and use low-fat cheese instead of regular.
SPICES, SPICES, SPICES. Try thyme on corn, cinnamon on carrots, anything to make a healthy dish that has started to bore you more interesting. Buy some spice mixtures. I never use regular black pepper anymore, because there are so many wild and fabulous peppers available, black pepper has become boring to me.
Use extra lean ground turkey instead of ground beef.
Replace white rice with brown rice. I haven't had any recipes that were not better when I did this, IMHO.
When making a tossed salad, make enough for a few days, but don't mix the ingredients except for what you actually use that day. Especially keep tomatos separate. This way, if one ingredient goes bad, you don't have to throw the whole salad out. The ingredients will all keep longer if not mixed with tomatos.