I finally did 100 pushups for the first time today!
A few months ago, I started using the schedule published on hundredpushups.com (someone mentioned it on Planet Apache).
It worked well for me. Even though I could not always follow the course (it took me longer than 6 weeks) there was always somewhere I could fit in on the schedule and feel like I was advancing.
I have also been swimming several kilometers a week for the past few months, enjoying it and improving there too.
Eating more, loosing fat and putting on muscle, it is doing me a power of good.
(I hope this encourages you)