I'd suggest that only doing pushups every other day helps, like those before me have mentioned, your shoulders and arms take longer to recover than say your abs do.
Also, I'd kinda disagree with just doing pushups as fast as possible, doing any exercise like that will just tire you out too fast, just like with running you should establish a pace and stick to it. Personally I go for about one per second, theroetically then you could do 120 in a PRT and last time I checked that's plenty lol