Probably the biggest factor in boot time is the number of services that are started running. Find out what services are starting at boot time, and disable the ones you don't need (try running chkconfig --list to see some of them).
Another thing that can make a difference is the loading of modules. If you feel daring, you can recompile your kernel to include some of the stuff you are currently loading modules for (so they will be built-in to the kernel). Just recompiling the kernel can make a difference in boot time.
And best of all, if you get to using Linux all the time, you won't even need to reboot anymore, unless you add new hardware