Clothes pins (almost a cliche)
refrigerator magnetic note clips
paper clips/clamps (for stacks of paper--VERY tight, almost crushing)
Take a 5cc syringe and cut off the needle end. Flame the cut end with a torch to polish it so the areola and nipple skin isn't irritated or abraded. Go to the hardware store and buy a compression spring about 2-2 1/2 inches long and 3/4 inches in diameter so it fits over the plunger but does NOT fit into the barrel. Apply some vaseline, crisco, KY or whatever slightly viscous lube you like to the nipple and areola area. Place the spring over the plunger and insert the plunger into the barrel and depress the plunger all the way. Place it over the nipple and release the plunger. The spring will create continuous suction on the nipple. The stronger the spring, the greater the suction. (5cc syringe is the right size for most women and me, the 3cc is too small and the 12cc is too large)
hair clips
screw clamps (crushing pressure but get loose as they are worn)
spring clamps (medium or tight depending on its spring, but maintain tightness as they are worn)
small rubber bands (around the nipple)
ice --- https://www.youtube.com/watch?v=-mqbRDwCfHQ
candle wax
small titty floggers
piercing forceps - locking type (for grasping the nipple, I don't like piercings)
Needles are always good if you like to pierce. I don't. I have seen knitting needles used skewering both breasts with a single knitting needle.
Nails. I have seen nipples nailed to a 2 by 4--not mine and not by me.
On the softer side, a #10 sable brush is smooth a feels nice, almost a tickle.
I almost forgot feathers.