Original picture from picslutwhore
Get an old bra and cut holes just big enough for your nipples to pass through. Put on a top over it and spend the day like it with your nipples rubbing against yoru top.
To make it a little more challenging, lightly tie strong around your nipples, not tight enough to cut off the blood flow but enough to keep them pronounced to further irritate them.
I do love this idea and have often done it, but OMG I can’t watch this video without wincing. The way to do this without possibly cutting your nipples off is to use a marker pen to mark the spot and then cut the holes when the bra is removed!