Everyone keeps saying to never limp. While this is not terrible advice, I think limping can be fine at times. For example, I play regularly in a pretty soft, live game. I find it to be very profitable to limp with small to med pocket pairs to set mine. If I were playing against very good players I would do this much less often, but in this particular game it's a very effective move.
As far as the subject of mixing it up goes, I think in most cases it's a way over-valued concept. In higher stakes vs good opponents it's important, but it's too often an excuse to do stupid things. You should try to play every hand in a way that makes sense on it's own. Also, there's usually more than one good line that can be taken with a hand. You can mix it up and be less predictable without doing minus ev plays.