When I started playing as a child, my mother got me some lessons and so I've always served with a continental grip and decent if unremarkable form. During high school my first serve was probably one of the stronger parts of my game, but my second serve didn't have much action on it - more of a lower pace slice version of a first serve.
Since I started playing again after a long break my serve has progressed but not in a straight line. For the first six months I had decent looking form but no pace and little directional control, though I started to be able to hit more topspin on the 2nd due to watching pro video on tennisplayer and lessons on fuzzy. Finally, my form clicked and my pace jumped about 20mph in a week. I started bringing 110mph plus flat serves when my timing was right and went to an offset abbreviated motion with a shorter toss (Roddick influence). This went fine for a few years until I started getting more pace again. For about three months my pace jumped and I was sometimes hitting flat serve bombs over 120mph that actually landed in the court. Of course, I went too hard and ended up straining my shoulder, and locking up muscles in my back and neck. A year or so of rehab, seeing the chiro, and getting deep muscle massage made serving possible again.
I'm training again, but working more on form and placement. My advice is to always warm up and consistently do exercises to strengthen the shoulder area (thrower's ten, scapular pushups, pull-ups and deadlifts, etc). As I get older, I definitely am finding that I need to do off-court work if I want to keep playing tennis without injury.