First of all, you DO have a very nice ass and I really don't see any need for "improvememnt." However, IMO, squats are the ticket. To some degree, your knees can get used to them, especially if you find the right position for your feet. For me, having my feet at about shoulder width pointing out works best. Another trick that has helped me and my partner is mentally focusing on using your glutes to do the lift. That seems to reduce strain on the knees. Finally, if you really want to gain, doing "negatives" will really help a lot. What I mean is, you have a workout partner stand behind you and, once you can't lift again have him/her help you with the lift a couple more times with you controlling the downward motion. There are two ways I have done this and seen it done. The workout partner squats behind you and extends his/her arms to grab your waist and lift. The more fun way is the workout partner stands close behind you, squats with you and reaches around your body and locks his/her arms around you then helps you lift. When I lived in Florida, this was the most common method. Here in northern california, no one even uses spotters. Sigh.
This is something I would gladly help you with.