I believe the readme file included with the download includes links to some of the caching solutions but here's one for wp-super-cache which I believe is what most of us are using:
http://wordpress.org/extend/plugins/wp-super-cache/
Some folks also suggest moving as many of the plugins out of mu-plugins and into the plugins subdirectory and telling endusers to only turn them on if needed and only the ones that they need. While I understand that suggestion, I think it makes it harder for endusers to understand.
A decent box or VPS with a decent amount of memory is also a plus. Even a small wpmu install isn't recommended for a shared hosting account or with a host that oversells. If you have your own box, turning off unnecessary services is a big help.
You may want to search the forums for 'speed up' or other such terms. I'm sure this has been discussed previously.