Blast from the past: How to tell if it's a good recipe?

With the interest in making our own bath & body products rising, I'm seeing some really dubious recipes cropping up and I've been fielding a lot of questions from people about how to make them work.

Some recipes will never work because there are inherent problems in the formula. They might be using beeswax as an emulsifier, extreme amounts of emulsifier, or forgetting an important ingredient. They might be measuring their ingredients by volume instead of weight. Or they might be forgetting the preservative. So I'm resurrecting this post from June 2009 -  How to tell if it's a good recipe? - as a kind of checklist to which you can refer before spending your time and supplies on a recipe that won't work!