I am going to paint my guitar neck white. When I mask off the fretboard, how do I avoid the hard line that is caused by the tape?
Carefully placing your tape will get the line in a location where it is less visually noticeable, but regardless of where you have the line, you will probably be able to feel it. You can sand it with fine sandpaper (as fine as your patience will tolerate, 600, 1200, 2000) or fine steel wool. I find that if you use steel wool when the paint is only a few hours dry it goes pretty quick with very little pressure and therefore less noticeable scratching. Then the hard part, just buff it by hand, firmly with a cloth. Seems to be doing nothing at first. Depending on how gentle you were when sanding, there may be little need for buffing.

What kind of paint are you using?