John Saito, a designer at Dropbox, wrote down some very useful tips to improve the text in your UI.

Technically speaking, I’m a writer. I get paid to write words. But here’s something most people don’t know about me: I hate to read. ... You see, I mostly write interface text for apps and websites. It’s a style of writing where brevity beats brilliance, and every character counts. Writing interface text is actually a lot like design—designing words for people who hate to read.