The first thing to try is to boot into single user mode. This may not work, because your system might be configured to still ask for a root password to get to single user mode.
If that's the case, use another trick that replaces init with /bin/bash.
First, try single user. If you don't see either a LILO or GRUB boot screen, try hitting CTRL-X to get one (also ESC key can be used).
If it's LILO, just type "linux single" and that should do it (assuming that "linux" is the lilo label).
If GRUB, hit 'e', then select the "kernel" line, hit "e" again, and add " single" (or just " 1") to the end of the line. Press ENTER, and then "b" to boot.
You should get a fairly normal looking boot sequence except that it terminates a little early at a bash prompt. If you get a "Give root password for system maintenance", this isn't going to work, so see the "init" version below.
Another trick is to add "init=/bin/bash" (LILO "linux init=/bin/bash" or add it to the Grub "kernel" line). This will dump you to a bash prompt much earlier than single user mode, and a lot less has been initialized, mounted, etc.
Finally, you can boot your server from your favorite live/boot cd and use 'chroot' technique to reset the password.