How do I keep my seat from tilting back?
The problem is that there is only one bolt holding the seat at the angle you want.  Put enough torque on the back of the saddle and can overcome the friction that's holding the seat in place.

A few options:


Tighten up that bolt as much as you dare (but you've already tried that).
Increase the friction between the seat post and the bottom of the seat clamp -- either scuff it up or use some carbon assembly paste (ask your LBS, I've used it to keep my seat post from sliding down).
Switch to a seat post that uses a two-bolt design to hold the seat in place.  This is what my current and previous bikes used, one bolt pulls the nose down and the other tilt the seat back.  Get enough tension on the two and your seat stays right where you want it to.