My fat kittie cat...?

I have a 2 year old, medium haired calico female kitty. From the time she was 6 months old, she has been over weight. The one vet I took her to said it may just be her breed. She got sick(URI) a couple months ago and the other vet I took her to said that she could stand to lose a few lbs. (She's 18 lbs) How do you put a cat on a diet? Do you think 18 lbs is heavy for a 2 year old long haired cat? She isn't lathargic and seems pretty happy and content, especially when we have our groomer shave her coat down. Please advise...