I always make sure that my campaign contains 5 or 6 players, and I tell everyone we're running if we get four regardless.

Sometimes when I'm tired or bummed out, I don't really wanna travel to my friends' for tabletop, but I always found if I did go, I'd have a good time and be happy I forced myself to go - knowing the session will happen anyway is a great way to encourage people not to flake out.