A bit more intensive but accurate method:
Set up a video camera (I use a small Canon SD780 point & click digital) in line with the finish and with a good view of the start (often 10-30 meters to the side). Set a 10sec delay so memory isn't wasted while you prepare for the start. Run. Review if you want, but I typically just video my sprints and save the viewing for later on a bigger screen. Load the videos onto a computer and (I use Quicktime) find the time of start and time you cross the finish. Subtract for total run time. Quicktime reports in hundredths of a second, so with possible error and what not I estimate it's at least accurate to less than a tenth of a second. An additional benefit is video of your run for technique analysis.
I have also set stakes (road flares, as I had them handy..unlit of course, or light them for added pizzazz) at, for example, 15m, 30m, and 45m during a 60m sprint. Using quicktime it's easy to get splits at each of these distances. Might be overkill, but the data is there if I need it.
This is obviously more work than self-timing with a stopwatch, however it frees my mind from holding a watch and trying to push the button in time with crossing the finish. And short of deleting a video, you won't miss out on timing a run.