I have bad knees

hi, um just a little question thats troubling me. I have bad knees (surgery when i was younger), and i have a problem going on top. I cant last long without having to switch to a different position because my knees give out. I was wondering if i could get a couple tips or positions that i can try so i can go on top and be able to stay there until..... well you know that part. i could really use the help.

Thanks

P.S: please reply, i could really use the help