Trusty eHow has a page with nice tips.
My favourite are :
Read everything: cookbooks, newspapers, magazines, reviews, product inserts, maps, signs and catalogues.
Converse with everyone you come across. Chat with folks unlike yourself, from seniors to teens to tourists.
Keep a journal. Write down funny stories you hear, beautiful things you see, quotes, observations, shopping lists and calls you made.
Expand your horizons.

