Planning ahead to multi-boot Windows and Linux distributions is the way to do it, but what if you end up erasing your boot menu and losing access to Linux? The FOSSwire blog has you covered.
The open-source fans at FOSSwire detail the use of an Ubuntu live CD to get onto a system that Windows or another boot-hogging system has locked up and get things set back up. It's similar to how one would use GParted or the System Rescue CD to re-partition, but this guide offers a lot more step-by-step detail in how to restore a boot menu you had working fine before, and using an Ubuntu CD (or USB boot image) is probably a bit more comfortable for many than a pared-down partitioning tool.
Having recently rush-installed a copy of the Windows 7 Release Candidate and needing to get my Ubuntu desktop back, I found this link pretty invaluable. Got another hands-on guide for the suddenly boot-less? Post it up in the comments. Restoring an Overwritten GRUB Boot Loader [FOSSwire]