So You Wanna Administrate prgmr.com
Here's some handy commands for reference.
Need to know if a dom0 has sufficient RAM for a new user? Use xm info.
There is usually enough disk space, but if you need to find out anyway, you can use lvdisplay or vgdisplay
swapip.pl will let you grab a new IP for a user. Make sure they're not configured with an IP already - this doesn't change anything except what Freeside thinks their IP is.