I've never been in a real relationship type of relationship, but when breaking out of sitchations' I find myself in, I usually just say that I need to focus on myself right now and can't deal with any extra distractions (chile, I should be a work of perfection by now going on how many times I've been 'working on myself'). I do the gradual distance thing first, and then the whole it's not you it's me talk.
Honestly though, I don't think this is a nice thing to do to someone (gradually distancing without explanations). I think it's best to be direct in a nice manner and tell the person upfront what is going on. Especially if there are no major problems, you can end this amicably and with less hurt feelings.