One word:- Smudge Brush
OK so that's two words, and it's not that simple, but I create a selection following the contour of the area I want to fix and then use the smudge brush to clean it up. GIMP has the best tools for this by far, I do all my cleaning up work in GIMP and then use PhotoShop or PaintShop Photo Pro for any other editing. I like to use GIMP's point to point selection tool - it allows you to move each point before finalising the selection and enables you to very accurately position your selection. Also using antialiasing and a feathered edges value of 2 with your selection gives you nice smooth lines that are not too sharp. For the smudge brush I use a setting of 60% opacity and select the Fade Out option.
Hope that's helpful. I am working on a tutorial but it's a slow work in progress - just ask bmxmitch - I promised him a tutorial months ago and I still haven't finished it!