with the seatpost problem, if you can make the seatpost a little bit thicker it can help. You can put dirt, cheap (sticky) hair spray, piece of paper, tinfoil, etc... in there to make the tube grab it a little better. on one of my bikes i had a very creaky seatpost, it also slid down a bit even with the QR VERY tight. so i put some grease on the post, quite a bit of grease actually. this stopped the creaking and the sliding.
with the saddle clamp problem, in my experience, if it's just a little too loose and under a bump the saddle moves even one of those little notches, its all over. as the saddle starts to tilt backwards the screw gets more and more room to move, which means it gets easier and easier for it to move backwards.
in my experience, this can ruin a cheap seatpost, because your bodyweight grinds the little notches away until they don't grip as well. i was on a trail once and my polarized (or is it solarized?) sunglasses messed with the shadows, so i hit a large root i didn't see there, so my saddle took a lot of shock, and tilted back maybe 20 degrees. i got off the bike and noticed the clamp bolt was pretty loose, so i tightened it very very tight. it was ok for several bumps then did it again. after the second time the clamp was pretty useless, i could tighten it as hard as i could (and no dirt or grease in the clamp) and it would fall 90 degrees back just from trying to lightly sit on it.
i ended up getting a bontrager race (lite?) post with the clamp bolt out to the side, so I can adjust it easily on every ride without moving my saddle bag. so far i've had the post for about a year and it only did it once.