My wife has been on a diet the entire time we've been married.

The only thing I've found that works is that you have to have much more of any kind of carbs or meat, but you can otherwise make your dinner look like hers. You might need to add something that only you eat that she doesn't like. I find tossing down a can of Ensure Plus with breakfast helps maintain weight and no one wishes they also got some. I also only drink whole milk.

Also, eat out when she is not around. Snacking is the key to weight gain.