Since people are pushing mileage credit cards, here's one rule of thumb:

Never use a mileage card for charges that you won't pay off immediately.

In other words, never carry a balance on a mileage card. They invariably have high interest rates, and whatever mileage you earn will not be close to worth the cost in interest charges.

That said, for charges you do plan to pay off in full each month, it's a great way to earn extra miles for "free".