I'm no expert but ...

While some people with marginal credit may want to avoid signing up for and cancelling too many cards, I think most people without problems have little to fear. I've signed up for and eventually cancelled (usually just before the annual fee becomes due) every mileage earning card I ever heard of that offered a signup bonus and no annual fee or a waived annual fee for 6 mo. or a year. I've never had it negatively affect my credit, or at least not in any noticable way. I've refinanced without trouble. I also checked my credit score online recently and found nothing negative reported.

If you are worried about it, just stretch out the applications a bit to about one every six months, and instead of cancelling, take the bank up on offers to be swithed to a no-fee card when you call to cancel.

The one card worth getting and keeping IMHO is the Diners Club card for all the reasons stated on these boards many times. And before mktozd jumps into this thread with his sponsor code, let me say that me and most everyone else around here with a Diners card would love to refer you. I advocate giving the refrral bonuses to the Flyertalkers who post content most valuable to you.