It's best to use high quality clippers made for clipping toenails. I recommend Tweezerman Deluxe ($10). To avoid ingrown nails, cut nails straight across to just above the skin. Make sure your nail doesn't extend over the tip of your toe.
To get a soft square shape, file nails in one direction until they are even and slightly rounded at the corners. (Hint: Don't use metal files, they'll rip your nails). Basic emory boards like the ones from Revlon work great, but if you want to go truly pro, try the Mehaz ingrown toenail file which allows you to easily lift nail corners for shaping. Don't clip the sides of nails, that can cause ingrowns.
Emory board tip: The fine-grade surface is for smoothing the nail edge, while the coarser surface is for shortening and shaping nails.