You might check out www.waytorussia.net -- they have some good information.

Typically what you can do is get a travel agency to make a tourist invitaion for you for $30. This includes a token hotel reservation that you don't have to use. Once you get there, there are a couple of ways to work out the registration. The official way involves several forms and is described on the above site. The other way is to pay your inviting travel agent to work it out, for about $30-50.

They don't care about the specifics of the reservation, or that it was at the same place specified on your invitation, they just care that you got some sort of registration.

I used WayToRussia's service on my last trip and didn't have any troubles.