Welcome to Flyertalk.

Quickly, credit cards are a big factor. Look through the forums.
* The British Airlines card, if you qualify. 100K miles is quite a lot.
* Other credit card offers are the best for the buck, though you're limited by your credit score.
* Debit card offers aren't as lucrative, but are also pretty good, particularly for American.

If you're just interested in miles and trips for cheap, and not worried about one particular airline, you can take advantage of the super promos. They pop up from time to time. The idea is not to accumulate flying miles, but buy them for less than 1 cent.
This month, US Airways effectively lets you buy miles at about 0.75 cents per mile. This makes business class to Europe, 100K miles, under $800.
I'm sure there will be others.