good tips all around, and I would add - have a pen ready. And maybe a helpful friend. As soon as you remove yourself from whatever you're sitting on, feel around and pinpoint the deepest part of the probably rather large indentations you've made, and mark them with a pen. When I did this with paper on a yoga mat the indentations would disappear within seconds and be impossible to mark off. I also repeated the measurements several times, and reckoned an average.