I would say it's sort of like cutting wood. Measure twice, cut once.
Follow the instructions exactly, make sure the humidity and temperature are correct, and prop your piece up of the surface below.
You have to be exact when pouring in order to get the correct result.
Another thing is make sure you have a really tight boundary/border to keep the epoxy from seeping out. A minor gap will allow for the epoxy to leak out. The first pour I tested was on top of a scrap piece of drywall, my boundaries weren't really all that tight and it ended up seeping a bit onto the drywall underneath. It cured and was impossible to get off.
Again, prop it up slightly so that anything that leaks will drip onto the surface below. This way you can just sand down the underneath once it's all cured.
One last note, another good practice is to do a skim coat. Just pour out enough to fill in any voids or imperfections and to seal the surface. Doing this will get help with air bubbles forming in your final flood pour.
I'll hopefully be doing my final pour very soon so I'll post anything else I come across.