I've been doing push-ups for years, and a couple of years ago did 75, and never attempted more (I was scared to, quite frankly, afraid I could never top that), and this year decided I'd try for 100. The clincher was when I really paid strict attention to form - holding my core really strong, not letting anything "sag," not allowing my back to do what I call a "wave" motion on the way up, watching my hand placement - and it just clicked. Also, don't let your legs sag - that was a biggy for me to get over. Strong, strong core, and work with your upper body muscles. Imagine you have a board up your back.
Practice, practice, practice - lift your upper body, don't drag your back up (like you're dragging yourself out of a pool), I guess that's my best advice. Go back to wall pushups if that's what it takes to get your form right, then progress to chair pushups, knee pushups, then negatives - on the toes for the "down" motion, then on the knees for the "up" motion, next are the "military" variety that we all love!
Just Do It!