Gustavo’s advice to poll the Ubuntu patch archive for useful changes doesn’t have to be restricted to Ubuntu – for a long time I’ve regularly trawled at least the Red Hat/Fedora and SuSE packages for useful changes. The utility of doing this does vary with the package (typically inversely with the volume of upstream code changes that distributions tend to carry about) but for the time it takes to at least check to see if there’s anything interesting I find that it’s worthwhile.
Doing this is no substitute for getting patches submitted properly with good documentation and extraneous changes stripped out. Personally I have always been fortunate in the people working on my packages in Ubuntu – I’ve almost always found that the changes that are applicable to Debian have been sent to me already by the person making them. I’d go so far as to say that most of the substantial patches I’m given come from Ubuntu developers. In the one case this hadn’t happened the person making the change was surprised and apologized that they’d forgotten.