look for the hairclips that open up along a central spring, like a clamshell, have little fingers or teeth on the inside, and come in a variety of sizes- get small ones to wear under clothing (Great for giving hugs with-lol), and try the larger ones as well, till you find the combination of pressure/size that seems to suit you best. you can pick up a card of 20 or so in the dollar stores, so you can get as many as you like without breaking the budget; they can be quite decorative, too- and nipples are not the only places I find they work well,as labia/clit clamps as well, they do no damage, but work quite well.