Reddit mentions of Mainstays Multi-purpose Hanger Clips, 12 Piece

Hangers with clips (Either alone or on the hanger). On the hanger is fun because you can usually slide them towards or away from each other, and the tension will hold them a bit in place.

Vice grips - the weight can be really satisfying

Binder clips - even better are the ones with the magnet on the end!

Alligator cilps

Jumper cables, but probably need some heavy padding.

Chopsticks and tight (small) rubber bands.

Almost anything with "clip" or "clamp" in the name. So, for instance, if you have those heavy-duty clamps that get used to hold down a tablecloth at an outdoor picnic, that can be fun. Or woodworking clamps.

There are also things you can do to up the intensity even with gentle things. Ex: after the clothespins have been on a while, turn them 90 degrees. Or go more for the edge of the nipple rather than the base. Or get a spatula/paddle and slap your nipples to get them tender first.

You can really have fun with anything that squeezes. I remember as a kid using two clipboards. It was tough to get the nipple in, but was a fun sensation.

Hope that gives you some ideas!

Are you masochist at all? I've been known to clip myself with these when alone before. Just an idea~

These: http://www.amazon.com/Mainstays-Multi-purpose-Hanger-Clips-Piece/dp/B00GK8OVOC?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_detailpage_o04_s00

They have teeth so when removing them you want to be careful, so as to not tear off threads. I was a bit careless with adjusting one of the clips and it loosened a thread.

The clips are mounted to my wall with http://www.amazon.com/Gorilla-Glue-Heavy-Mounting-Black/dp/B019HT1U9E?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_detailpage_o04_s00 and mounting putty.