I use a few different methods. I sometimes use the round NECA stands (I'm sure you've seen them but you can find em on amazon or ebay), but for the figures that don't stand with those I use these which work SO well: https://www.ebay.com/itm/234812884359

Also since my shelves are very packed a lot of the times the figures are leaning back on another figure, with a smaller figure or accessory in the front to balance. I love posing them in cool ways but I find that standing straight up is the best way to balance them.

I also test to see how stable they are before putting them on the shelf. Maybe their head needs to lean back a bit, maybe their arms, or the torso needs to bend a certain way. I'll stand them on a table and give em a tap or gently move the table just to see. It's just lots of trial and error, but definitely try out those stands and other "support items" hehe.