This one's the for landscape/seascape photographers out there like myself who like to blend multiple exposures of a scene into one composite image (for dynamic range), and who have found that traditional selection tools are inadequate for tricky, detailed images.
Seascape photographer Brent Pearson has produced a series of three videos which shows you how to create and refine a high-quality mask using contrast rather than the usual selection tools.
Here are the entries on Brent's blog, from which the videos can be reached:
This is a technique I haven't yet used, but I'm going to give it a go on one of my images this afternoon. I'll post the results (assuming I succeed).