when i mix, i take regular breaks, as you can get too set into listening to what one part of it sounds like, taking a break freshens the ears and lets you revaluate.
i also tend to play it back through other speakers, or just step back and listen to it without trying to mix it.
i think the big tip is if you make a change, keep it small, for example, if you add reverb to a guitar, keep it minimal, as once you get too heavy handed it impacts on the rest of the mix, and youll be going round in circles trying to mix it all over again. generally i find the first mix is usually a good indicator, and tweaking should be kept minor after that.
above all, don't let it stress you, if you think too much on it you'll change things, when you change things it affects everything in the mix