"You can't polish a turd."
This means no matter how much you try, it's not getting any better. Most of the time, this refers to someone else's crap (no pun intended), or them personally, and is derogatory in nature. However, I use this phrase often when cleaning my house. That carpet might be vacuumed and free of clutter, but it's still stained and looks like hell. No matter how clean the windows, they still look out onto our yard, yet to recover completely from years of garbage storage with the previous tenants. No matter how much mowing and weed-whacking and flowers planted, the house in the center is missing siding and falling apart. No matter what I fill it with, or how clean it is, this place is still kind of a dump.
Sounds sort of cynical and pessimistic, but it's not. For me, this is a humorous way to appreciate what I have. Nope, it's not perfect. But I can't expect it to be. After all, you can't polish a turd. And that's ok. This turd is my home :)