I just read Steve Krug's Don't Make Me Think and while I really liked the whole book, what especially stuck out was the chapter on usability testing as often as possible.
It doesn't need to be an expensive or time intensive process - just grab 3 people for 45 minutes each, pay them 50$, show them your website or app and ask them to use it while thinking aloud. Maybe give them a few simples tasks to do.
Repeat this process regularly - say once a month - and you should be able to catch a significant fraction of the usability problems on your site very cheaply.
Tags: programming, ux