You can't make and freeze everything ahead. Sigh. Some stuff--like vegetables--don't like to get chilly, so here's a step-wise approach to preparing side dishes using fresh ingredients easily.
The better the produce you start with, the less work you'll need to do to make it taste good. Fresh is best, frozen can be pretty good, canned is almost always horrible. Just my $.02. I like making a side dish that doesn't require any extra ingredients; I really hate picking out produce and then tracking through the store to pick up the "extras" I'll need for elaborate vegetable recipes.
I usually plan side dishes that cook them the same way I'm preparing the main part of the meal. If I'm roasting a chicken, I'll roast some vegetables. If I'm grilling burgers, I'll grill the sides as well (grilled fruit makes an amazing side dish). If dinner is prepared on the stovetop, I'll steam, braise or boil my veggies. It's really about matching what you've already got planned to do the least amount of work possible.
Here are links to the 3-tier recipes I have for vegetables on the blog:
Mashed Potatoes (yes, this deserves its own entry)