Someone once posted here to smile when you're climbing hills. I have made that my motto, and I think it helps. Beyond that (and I'm a relatively new rider- just since last summer)- keep riding up the hills and use your low gears. Someone said to me, it doesn't get easier but you get better at it. I just plug away at it, and go back to the same hills regularly. I see my progress that way.

I mostly climb sitting down. If my legs have already started to get tired/sore, then I simply can't stand up. If I try standing up before I'm hurting, it works better for me.

Good luck!