Here are some tips that I have found helpful with my own cats:
1)control portion by scheduling their feeding times and removing always available feeding bowls.
2) purchase high quality food made specifically for overweight cats (that Innova Evo that lil_bit mentioned is what I feed my cats). Also, you can put a rubber ball in the middle of their dry food dish to eliminate "gorging" and slow the eating - this also helps with cats that cough up food.
3) buy a string toy and exercise your cat for 5-10 minutes a day.
4) limit treats to 2-3 a day. Treat them to catnip once in awhile! Catnip is a great treat that is low fat and boost their energy levels.
If no results, talk with your vet to see if any thyroid condition exists or other health conditions that may be keeping your cat from losing weight.