One of the most powerful tools that you can use both for
SEO and general site management is to get to grips with mod_rewrite
What does it do?
What doesn't it do!
Seriously though.. mod_rewrite allows you to control and rewrite URIs to suit your needs. You can turn ugly ?=45sd type junk into meaningful and more attractive URIs that both your readers and search engines can take advantage of
For some basic information check out the Apache documentation:
Apache module mod_rewrite - for Apache 1.3.*
Apache 1.3 URL Rewriting Guide - more detailed with examples
If you want to get your hands dirty then this site is an excellent resource:
mod_rewrite Cheat Sheet - Apache - ILoveJackDaniels.com