I've been using a "round" system in my RPs for the past three or four years and it pretty much has no order of posting but ensures every player posts once and only once per round. So I would post and then everyone else in the RP would post at their leisure, once and only once. And after everyone has been accounted for, I start the round again so no one is left out. It does take more time but I was sick of people flaking or being left behind so this tends to ensure people are active and engaging one another so it doesn't become a fuckfest for two people or that someone ends up missing a big scene because of some personal issue offline. Sometimes a posting order can develop with this method but I don't encourage it because who needs that kind of pressure?