Mix it up to increase pain level with a variety of sensations to make it more sensitive.
Abrasion of nipples with scotch brite side of sponge, sand paper, bristle side of a hair brush etc.
clamping in dif directions with dif implements. After being clamped for about 5 minutes remove them and rotate 90 degrees to allow clamping in a dif direction. If nipple was a clock face you’d have clamp points at 6
And 12 but then after 5 minutes rotation would make the clamp points at 9 and 3. After a rotation and 5 more minutes. Give them 5 minutes of rest then repeat. This will allow time for blood to return to them and make them more sensitive.
As far as different implements use sharp roach clip style clamps, clovers, chopsticks/pencils rubber band method, rubber banding nipples only to increase sensitivity, carpentry wood clamps, pliers, jumper cables etc. they all will give different feelings and effects.
Band snaps and or impact to nipples.
Banding or binding Tits at the base to make them tighter.
Hot cold play with wax ice water fire etc.
These can all be done separate or together. Mixing in some of the actions will help bring more pain.
For example clamp session with rotation. Impact. Abrasives. Back to clamping.