Don't use the red inserts, don't buy 5 minute epoxy and don't store the two bottles in a single zip-loc or other sealed plastic bag. I keep mine in a cupboard in my basement (seperated a few inches) and they last years. I only use 30 minute and finishing resin epoxy. The faster kinds seem to "kick" all on their own.

If it does sieze up on you. Boil some water, remove it from the heat, immerse the sealed epoxy bottles in it. They will thin out. Don't use them until they cool unless you need a very thin and runny epoxy.