i keep my creativity going in a lot of ways.
first, i choose to run my own studio. after working for other people and a few in-house jobs for a number of years, i have discovered i prefer the varying clients and relative creative control i have by running my own shop. my partner and i right now have 4 large active clients that we juggle around every week to get all the work done and another 5 or 6 that are ongoing and in the works (i am worried a ton of work will hit at one time, in which case i may have to hire another designer to cover it all). now the point is by having varying clients i can get some good "bread and butter" work - high paying, reasonably straightforward work which allows me to take on other, lower paying but much more creative jobs. and then sometimes you get high-paying, highly creative work which is the best of both worlds. the idea is to have a mix of work so you can use varying parts of your brain in any given week. if you have a regular jobby job then some freelance is a good way to have this situation without giving up the security of a regular paycheck. having said all that running your own show is a lot of stress, a lot of time spent NOT being a designer but being an accountant, a billing machine, a fedex deliveryman, etc... but at the end of the day its worth it.
the other thing i do is teach, which probably gives me more intellectual stimulus than all my clients put together. the catch with teaching is it pays poorly (relative to client work) but having the dialogs with students keep smy mind sharp and open to new things.
you also need to have a professional dialog, and by that i mean read the trade magazines (tho i think HOW is possibly the worst magazine on earth for inspiration), go on to design observer, speak up, typophile, etc. go to design lectures at any schools near you. get into the conversation. you need to be talking, if not literally than at least visually, with other designers/design minded people. the worst designers are those who live and work in a vacuum.
you also need to have a contextual dialog. go to art museums. read the newspaper. go to a concert. you need to understand and embrace the context that your work lives in. you will gain inspiration by being a part of the context your work should be talking to.
you can also use iterative processes to help spark something. carry a small camera with you at all times and make yourself take 50 pictures a day. make yourself do one one-minute drawing a day during lunch. force yourself to spend 10 minutes every day for a month making something from a piece of paper, a gluestick and 5 paperclips. take a newspaper article and make 100 xerox copies of it and see how creative you can be within the medium of a copy machine. sounds silly, but this kind of forced process can actually make creativity happen because you are not giving it any other choice.