You can increase your mileage yield from CC spending by shopping through the portals, especially with CO and their "Double the Miles" promo. I'm averaging just over 3 miles per dollar on CC spends.
Examples:
- Petco $13, 10x the miles via Shop Onepass = 143 miles (130+13)
- 1-800-Contacts, $135, 8x the miles = 1215 miles
- Sears, $239, 10x the miles, 2629 miles
- Drugstore.com gift cards, $500, 10x the miles = 5500 miles
- 1-800-Flowers, $75, 50x the miles = 3825 miles
Since March I've put around $12k on the card, and netted just under 40k miles.