Polyglot, why don't you post an example.... or link us to one you've already posted...

RE: preflashing, I usually do three stops under (=shorter than) the normal exposure.

Note also that with split grade technique you can manipulate any part of the tone curve. But... pardon the insult if it is one... maybe the issue is more to do with your negs? Consider where the details are falling on the density/exposure curve of the film. If the highlights that you'd like to see in the print are way up on the knee, then they will clump/posterize unless you work quite hard in the print phase. If you lower your exposure a bit, some of that highlight detail will slide down along the shin of the curve where it has more slope... hence better highlight differentiation. Of course, shadows will also slide more into the toe, but from what you describe, you can afford that.

RE: Marco's comment about fiber vs. RC, a big issue for any newcomers to FB is the drydown and how much that changes the apparent contrast. I go at it two ways: (1) print a grade or so more contrasty on FB or at least "bracket" the contrast on a test print that I can microwave and quickly see the result; (2) overexpose the FB print and bleach back a bit. Though this method I find that I can get about as much highlight zing as I could ever want.