I've been using trackpads for over 11 years, all of which have been Apple products. Of the three laptops I currently use, all have mice available to them. If I'm doing something requiring lots of movement, I'll plug the mice in. When I first got my iBook, I used the mouse more than the trackpad. In fact, I took it with me when I traveled with the computer. This allowed for me to transition to the trackpad whenever I felt comfortable, building up my fingers in the process. Nowadays, I don't use the mice much, but they do get plugged in if I'm going to be doing a major project (like an iMovie).
I tend to go back to my Apple II roots quite often with computers. I use arrow keys to navigate webpages and utilize keyboard shortcuts whenever possible. I think this, combined with my mouse tendencies, takes a strain off the fingers. I've never had a problem with sore fingers, and I think varying the input device is key here.
Osxx mentions pressing down hard. I just run my finger over it and barely touch it whenever I need to click. It's almost effortless on the newest trackpads, and even with older, mechanical button trackpads, you don't need much force at all.