One of the best ways to learn to let your imagination flow is to try and think like someone... or even something... else. The art of RP allows just that. RP means Role-Playing, and no I'm not talking about a session with your counselor! ;) I'm talking about a game. Otherwise known as RPG, or Role-Play Gaming. This comes in many forms. The most simplistic is a game like "Zelda:A Link to the Past" or "Final Fantasy" on your handy-dandy Super Nintendo, or other home game playing system.
Ok, you don't have to really think much to do this (as far as imagining to be someone else goes, that is, the games are fairly indepth and do make you think) but its the first step on the amazing upward spiral of RP'ing. If you enjoyed being someone else for a little while, come on-line and find one of the many on-line multi-player RP games available,in which you actually control who and what your character is and does. Go back to my home page for a list of some that I have beta-tested and read their descriptions.
If you would prefer to see the people you are rp'ing with, go to your local hobby shop and inquire about D&D games ongoing in your area. Many hobby shops host these games themselves, and you will find there are many conventions and gaming shows that are centered around this genre.
I started rp'ing long ago when when we all just stood (or sat) around a table with a bunch of funny looking dice and created characters accordingly. Nowadays, there are books and boxes and rules that, to my way of thinking, are more of a hindrance to imagination than an aid, but thats MY opinion, hehe.
There are also online RP games that are strictly text (some are like a bulletin board, some go on in chat rooms) that are more the "thinking" type games. Try them all and see what you like!
Of course, as a writer, I think simply taking a pad of paper (or your keyboard) and making your own story is a great release of imagination and a wonderful kind of RP since you must have characters of different types and must "get into their heads" so to speak, in order to make them work! :)