JamieJo - I found progressing from angled push-ups better. The problems I had with knee push-ups were weight related. ie. they killed my knees.
Back in probably March I started with 60-degree (treadmill handles), moved to 45-degree ones (desk), never found a surface I liked for 30-degree ones though. I can (relatively) easily work in 4x6 into my strength training routine of full push-ups now. I've done as many as 4x15 at times, but that's still pretty tough at my weight.
This afternoon I'll be trying decline (or incline depending on how people consider them) where I'll put my feet on a bench... well, my couch. I've done some of these before (and they're tough), however, my routine goes to 6,4,3,2 today and I figured that if the 4x6 weren't killing me it was time to try something harder.
