clipnodes are one of the few things that you can reduce without impacting the map too much simply by filling in (or covering up) bits of the map that the player can't reach with clip brushes.
inset lights, unreachable terrain and such.
that said, i've rarely hit max clipnodes before hitting other things like max vertices.
if you do find yourself getting errors,
http://user.tninet.se/~xir870k/tooltips.txt is a great resource for figuring out what is causing them.