I read a really good article on increasing the # of pull ups/chin ups you can do (I'd provide the link, but I didn't bookmark it). In any event, some of what the author said can be used as advice for pushups, too. He said the only way to get better at pull ups is to do pull ups. He said, sure one-arm rows will increase back strength, but don't expect to do a bunch of those and then magically crank out 12 pull ups. So I think the best way to do pushups is to just keep doing them, whether on our knees or granny style, just keep doing them and next month or so, your #s will increase. And thanks to Cathe, there is no shortage of pushups in mesocycle 1!!
To tell the truth, I was really pissed at myself when I couldn't finish all the pushups from Disc 1 on my toes (heck, I couldn't finish the '30' reps on my knees!) but then I thought about it and I'm glad I didn't get it my first time out - it gives me a goal and a challenge to work towards.
As for the straddle pushups - I have the toughest time with those, but I find that the wider apart I have my feet, the easier they are.
Also, another trick is to do the pushups in a Low Impact Circuit sort of fashion - in that workout Cathe does the up motion on the knees, then the low motion on the toes, and vice versa - I think this helps. I try to do triceps pushups in this fashion because a straight set of on-the-toes triceps pushups, for me, is too tough. For now.