I had the same problem on my Trek MTB. I'm heavy, so bumpy rides would make the seat slide down. However, if I tightened the nut enough to where it wouldn't slip at all, I couldn't close it without stepping on it. Then I couldn't open it by myself.

BUT, my qr has an Allen nut on the other side, so I just crank the quick release lever to hand tight, and then tighten the nut just enough with my multitool. It sucks having to use the allen wrench when I want to lower the seat to descend, but that's better than it slipping down slowly or suddenly during a ride!

I have to step on the quick releases on my wheels, too. My hands are just not that strong.

Karen