Doing 100 is cool. But I've figured out how to do 30 and really get you drained... have some fun with the pushups here's my 2 cents please read as it is useful for gains.

I do what I consider a perfect push up. and my nose touches the ground and my midsection is stiff as a board and I feel my abs. However... it's fun to try to touch your nose down an inch higher the next pushup and after that two inches the next one until the fourth is the good pushup you started with. That way I find you're constantly challenging your stabilizers and making the rest of your muscle groups work. Let's face it, once you've got 100 done daily for a while some challenge can be fruitful, and pushups are boring so why not tap into your will power? it's worth it if you want to be worth the potential. Like I said, my 2 cents.