In reply to purplemonkeyelephant:
I spend about 3 months a year in Russia at present. I'm actually just back from Sakhalin now.
The rules used to be that you had to register your visa within 3 days of arrival. It's now 7 days. If you're travelling around then it's best to keep your itinerary with you, including copies of any hostel, hotel etc booking confirmations or tickets to/from wherever you're travelling, as evidence that you're moving about.
Places that see a lot of foreign visitors can often do a registration overnight. For instance, where I stay in Sakhalin, they don't even take my passport....just photocopy it, my visa and entry/exit form and by about 1000 next morning my registration's back. In Moscow, my normal hotel keeps my passport overnight.
I'd suggest that if you're anywhere for more than 1 night, you get registered to be on the safe side...so even at your next destination you can pull out the "expired" registration from your last city as evidence.
In fact, I've never heard of anyone having significant problems with the registration side of things. I have known people to have BIG problems on exiting the country when they've lost their entry/exit form. This is a white form that's stamped on entry. Most places you enter now, you don't have to actually fill a form out manyally any more, it's generated and printed at the immigration control desk on arrival. DON'T LOSE IT. If you do it could cause a MAJOR headache on leaving the country. I clip mine into my passport on the visa page with a paper clip. If I wasn't going in and out so much I'd probably staple it!