I find that if I set myself up by thinking of "widening" my back and chest, I am more stable for push-ups.
The gap between "on-the-knees" and "on-the-toes" push-ups seems so big. I try to do all I can on my toes (I got to do a very good amount with P90X), then I go to one knee on a balance disc (the other leg held off the ground), then to two knees on a balance disc.
Oddly enough--or it seems so to me--I can often do more push-ups on my toes with one foot stacked on the other than I can with both feet down. Can anybody tell me why? Seems like it would be harder, but for me, it isn't.
I'd also say experiment with foot width: it may be easier for you to do the "on-the-toes" push-ups with a wider stance.
Also, doing "negative push-ups" (starting in the "up" postion on your toes, then S-L-O-W-L-Y lower down, drop your knees down, and push up slowly to the top.
Give yourself as much time as necessary between sets to regain some strength (but remember "it's a break, not a vacation"!). I cannot do all the push-ups at the beginning of GS Chest/triceps with the short rest that is in the workout. But if I recover a bit more (maybe with age comes more of a need for recovery!), I can do most of them on my toes.