The important thing is to come up with a strategy which is self-consistent. Always open-raising is an easy strategy to implement this because you know you're not giving your hand away.
You may be able to improve your results by limping in certain situations. One way to go about this would be to figure out what hands you want to open limp with and then balance that with certain strong hands. That way when you open limp your opponent will know you're either relatively weak or very strong, but won't know which.
Abdul has posted some interesting ideas which talk about this in a more sophisticated fashion (plus look around at the other pages on the site talking about pre-flop play)
http://www.posev.com/poker/holdem/st...ance-abdul.txt