I created a /etc/profile.d/ulimit.sh file containing the following:
ulimit -u 300
If you have a real multiuser setup and allow users to choose their own shells you should also create the equivalent ulimit.csh file with the appropriate c-shell command instead (I believe c shells use limit instead of ulimit...`limit maxproc 300` might work but I have no experience with those shells). I use a lightweight window manager and I haven't encountered problems with 300 processes per user, but you may with the heavier stuff (though I occasionally run KDE without problems as well).
under a heavy session would give you a starting point as to how many processes is reasonable. I could probably get away with 100 but 300 is enough to provide adequate freedom while limiting the impact of fork bombs so I'm content.