Last spring I was 263 pounds and couldn't get up hills very well at all. I ended up walking a lot of them. I'm down to about 247 and, with practice, I'm flying up hills.

Well, I'm flying up some hills. I did have to walk up one this weekend.

But practice is the key. Downshifting is good, but it won't make up for endurance. What I did was make sure there was at least one big hill in every ride I took. I'd get the bike in a hard to pedal gear and get up speed before I hit the hill, then I'd slowly downshift until I either hit the top or got off and walked.