I use a very tenacious double sided tape used by wood turners from my local Woodcrafters store. It's quite strong and as Mike said, be careful prying a flexible object off of the tape. Acetone or alcohol helps remove it. I have a steel plate glued to a hardwood block under the tape and it reduces any vibration. Usually, I can remove the object with a thin, sharp, wide wood chisel slid under the object slicing thru the tape.
I also use the woodturners tape from Woodcraft. I tape small brass practice plates to a 1/2 thick steel plate, that I then clamp in the vise. I use a heat gun to loosen the bond and then use a wood chisel to pry it up. For practice plates, I can use the tape several times. Oddly, I also use this tape for wood turning, where the biggest issue is separating the work piece from the face plate.
That plastic stuff that was mentioned..........looks like a product called Jett Sett. I tried it many years ago and it was okay sort of. But water and steel do not mix and it was a pain. I've been using Thermolock ever since as it was easier and less messy and of course, no water to rust steel parts!!!
I agree Andrew - I experimented with it out of curiosity, but the very idea of locking some water between the Jett and a finely polished metal surface gave me the jitters - just too much possibility for rust. I often leave Thermolock in place for months while I am doing various different parts of a firearm.
i occasionally use 2-sided tape to hold a part to a thermo-loc formed fixture block. put masking tape on the part where you'll want to use the 2-sided tape to account for the thickness when forming the warm plastic. this combo has largely taken the place of the MUCH more time consuming, MUCH less convenient bondo and pitch. have also put masking tape over the formed, cool thermoloc and used hot glue for attaching part (can't use hot glue directly on fixturing plastics).