One of my most popular blog entries of all time is my Contracting Tips: Fixed Bid vs. T&M post from January, 2004. This post consistently shows up in my referrers list, usually coming from a search engine. Recently, Brent Ozar (@BrentO) wrote a great argument for why he always bills by the hour (a.k.a. Time & Materials or T&M) which itself was a response to Mark Richman’s (@mrichman) post on why he never bills by the hour (fixed bid). Each article has good arguments, and I encourage you to read them both and choose the best approach for you.
As for me, my experience parallels Brent’s and I historically have leaned toward the Time & Materials model.