1. You signed up early for a Google Wave account with this request form. If you didn’t fill it out, we recommend you do it now for the next wave of invites. (see what we did there?)
2. You have an account on the Developer Preview of Wave.
3. Some paying users of Google apps will get accounts
4. You are invited by someone currently using wave.
“We’ll ask some of these early users to nominate people they know also to receive early invitations — Google Wave is a lot more useful if your friends, family and colleagues have it too. This, of course, will just be the beginning. If all goes well we will soon be inviting many more to try out Google Wave.”
Since a majority of the invites have been sent out, your best bet is to see if any family or friends got invites, and to ask them to send you one.