I once dug a 2 foot wide, 4 foot deep trench all around one of the gardens. It's the only sure way to keep woodchucks out, and they haven't been in the garden since.
Other things:
Some people put a fence about a foot in the ground. I don't know how effective that is though because I've seen them dig far deeper than a foot.
They can climb as well, fence should be at least 3 ft tall.
Trapping helps, but you just can't get them all.
Planting something they can't resist (like broccoli) outside the garden.
Mothballs: These don't work. I read in several articles that mothballs will keep any animal out of your garden. This is not true. Been there, done that, didn't help. I bought all the mothballs I could find and threw them all around my garden. The smell was so bad that I could only work in the garden for a few minutes at a time. Yet, the woodchucks came in, so don't believe that this stuff helps.
A dog. At least you'd hope so :)
Raised beds. Here too, I'm not sure if that is true or not, but they say that woodchucks are less likely to eat your vegs when they are planted in raised beds. (They = the vegs, not the woodchucks)
