Read a WPMU Signup page carefully... you know like those "Spot the Differences" riddle we used to solve when we are young tots. You should be able to come up with at least 10 combinations. I was able to get more than 25.
Then use each of those with some popular domain extensions (i.e. TLDs) e.g. com, net, us, org, biz, info etc. and you've just multiplied your footprints.
Total Footprints = Footprints * Total-TLDs
So, you have...
site:com "(We'll send your password to this address, so triple-check it.)"
site:net "(We'll send your password to this address, so triple-check it.)"
site: org "(We'll send your password to this address, so triple-check it.)"
....and so on.