How do I help tidy up my elderly parents home without a confrontation?

I'm facing the problem that my parents (both in there 70's) live in a very cluttered old country house. Though my parents have been wanting to move to town for years, there is always an excuse as to why they can't (mortgage needs to be paid, we need to clean house before we sell it, what to do with the dog, etc.). It costs a fortune to heat the house in the winter and there are so many repairs needing to be made that can't be fixed due to a low income. We've tried tidying up the house but it's very difficult. My dad doesn't know how to clean up after himself (he was a bachelor up until his 40's and just drops his clothes, lottery tickets, etc. everywhere). My dad does most of the main things like yard work, laundry and cooking for him and my mom, but feels he does not have to clean up after himself around the house. My mom is due for another hip replacement later this month. She still works (though it has decreased dramatically in the past couple of years), but due to other recent surgeries and heart problems she can't do much around the house. I am an only child but I've been living in town the last few years, going to college and trying to find employment. I've tried cleaning up the house for them many many times, but my mom has so many "sentimental items" that she gets mad at me if I try getting rid of any of the clutter (yet they complain about the clutter all of the time). This is also with items she hasn't worn, used, read or looked at in over ten years. If I mention the clutter or ways to try to improve it at all my mom immediately snaps at me. After her surgery she's going to need more room in the house for mobility reasons, however in order to tidy up the house at least 50% of the clutter will have to go. I am at a loss for what to do. The other problem is with my extended unemployment I will have to move back in again. I'll have to be here more after my mom's surgery anyways, but every time I move back there I get extremely depressed and my asthma kicks into high gear. It seems all of the house cleaning is left to me because my parents won't do it, yet with the issue of my dad throwing his stuff everywhere everyday and my mom refusing to get rid of anything (except actual garbage), it's a never ending battle.