I have two suggestions.
1. Put fitted sheets on diagonally... picture an "X" on the bed. Start at the hardest corner (pull the mattress down so you have room for your hands to get around and under the corner... lie on top of the mattress if that makes it easier.) Now follow the "X" and do the OPPOSITE corner. Then do the next hardest corner (that would be the 3rd corner ) and then do the remaining corner.
2. If your fitted corners are constantly slipping... you might try elastic snaps. Get them at Sears. They are like mini suspenders with clamps at both ends. Just clamp them on your fitted sheet before you put the fitted sheet on the corners.