I do a lot of this kind of thing, thanks to this board, actually.
Someone posted a link years ago to Once A Month Mom, which gives a listing each month of freezer meals. Great ideas, great recipes, and things you can start with and branch off from. You can find a lot of great ideas for casseroles and such on there.
If you wrap things in plastic and then foil and also keep them in a separate freezer, they'll last a little longer. We found that keeping things in the top freezer portion of our refrigerator makes them more prone to freezer burn than keeping them in the huge freezer we have in the basement.
I know you asked re. casseroles, but other things I do...
1. Boil up a batch of ground beef when it's on sale. Drain it. Separate it into smaller bags and then at the last minute, we can just toss it into whatever we're having that night for dinner.
2. Instead of buying chicken salads at work, I'll do what another posted suggested and boil up breasts in advance, but I cut them into salad-sized portions, put them on a cookie sheet lined with foil that's been sprayed with cooking spray and flash freeze them. Once they're frozen, pop them into a storage bag and you can take out as much as you need.
3. Freeze home made french fries. Peel potatoes and cut into french fries. Shake the raw french fries in a little olive oil and salt (or any other seasoning you like; I sometimes use Old Bay). Bake at 425 until just done (maybe 8-10 minutes; depends on how thick you cut them). Cool them on the cookie sheet on which you baked them and then set the cookie sheet in the freezer until they are frozen solid. Scoop them off with a spatula and store them in a zip lock bags. To heat them up just throw them back in the oven on a cookie sheet at 425°F for about 15 minutes.
Check out Once A Month Mom. There are tons of great recipes on there.