I have recently started using TENS. The electrode pads don't stay sticky for too many uses so I tried a few things to see if I could extend the life of the electrodes. The best one was clear 2 inch wide packing tape. The cheap stuff that I bought for sealing boxes when someone moves. I place the electrodes sticky side onto the sticky side of the tape.
I have a cheap plastic two pocket folder that they sell for back to school and I stick the tape that surrounds the pads on the folder. When I want to use them the next time I just pull them off the tape and they have enough adhesive to stick for another day. I think this will add a lot of use to the electrodes before I have to replace them.
Make sure to fold a bit of tape at the end of the roll back onto itself so you can get it started easily the next time you need tape. The adhesive on the tape is quite mild and it hasn't bothered my skin at all so far.