There are at least two ways to take a star trails shot - take one really long exposure or take lots of exposures and stack them on each other. I've done both and prefer the look of the latter method.
Below is the result of that latter process - 187 exposures, 22 seconds each. I've applied a "comet" effect in Photoshop by grouping exposures and graduating the opacity of the groups.
How do you prefer to do your star trails?