I use several different contextual menu item plugins, including PicturePopCM (which allows you to preview pictures and pdfs in the Finder with a "Show with PicturePop" menu item), OnMyCommandCM, FileUtilsCM, and QuickAccessCM. All of these plugins are in my Contextual Menu Items folder in ~/Library.
I noticed that when I control-click on an item, the order that these items were listed in the contextual menu corresponded to the order that the plugins were listed in the Finder (in list view). I wanted "Show with PicturePop" to be the topmost contextual menu item shown, so on a hunch, I renamed the PicturePopCMPlugIn.plugin file with a space in front of the name. As I hoped, this caused it to appear at the top of the Finder list, and at the top of the contextual menu.
This hint should work with any CM plugin that resides in your Contextual Menu Items folder, not just the ones I used.
[robg adds: I have not tested this one myself.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030413113037892