I guess the first thing to do is to actually care about it to begin with. I have tried submitting suggestions for improving UIs (and even more trivial bug reports like some element is not centred properly or a text box is 1 px too tall) and it seems like many developers simply don't want to hear about it. They don't have an eye for it, they don't think about how other people might approach the interface, so it just doesn't bother them. They consider your criticisms and suggestions to be nit-picking or pedantry and they either get defensive or they just dismiss them outright. Even if they don't resent them, they often consider them to be so low-priority that they're not worth putting much effort into.

The second thing would probably be, like others have said, just to ask -- and part of asking for help is making sure that you are able to actually receive the help. Perhaps partially because of the above, and also because i am not really a programmer (yet!), i have come to be hesitant to even try submitting anything. The process is too complex. I feel very uncomfortable with patches and pull requests and test cases, so please don't make me deal with them. Offer a 'dumbed down' way of submitting our contributions, or, perhaps, make somebody a liaison between people like me and people like your developers.

Large open-source projects in particular -- Firefox, GNOME, KDE -- are ones that i have the most interest in contributing to, but they have extremely high barriers of entry for people like me. (To a relative outsider it kind of seems like the design direction of these types of applications/suites is strictly controlled by some high-up cabal of developers, though, so perhaps that is intentional.)

edit: wording