I find a lot of people from all walks of life don't fully understand Frequent Flyer Programs, how they work or how to make them work for them. I'll admit that even I am just getting a grasp on them.
The first misconception about frequent flyer programs is the idea that you need to fly a lot with one airline to take advantage of them, and while that does help it's not the only way to make them work for you.
Most airlines are part of airline alliances, this allows you to fly on one airline but build up points with another airlines frequent flyer program. For example: If you belong to American Airlines (AA) frequent flyer program and book a trip with British Airways, (BA) both members of One World, you can use your AA program number to buildup your points from your flight with BA. Or, you can use your AA points to book your flight with BA. With this understanding I try to keep a frequent flyer program with one airline in each alliance and use that number when I fly with other airlines within the alliance.
Building up miles can be quite difficult if you don't fly too often or fly with whoever has the best deal. If you're using the same airline or airlines within an alliance most of the time, but not enough to earn your miles, then you might want to consider getting thier credit card. You'll earn points on your purchases and build up your frequent flyer account more quickly, maybe just enough to get an upgrade or free flight. Take a look at different opportunities to earn double or triple miles with your credit card, it may take nothing more then using your card at the right places. Be aware that credit cards associated with frequent flyer programs often have large fees - from no fee to over $100 a year. BUT, you may be able to earn these fees back by having the card. One of the benefits of the Continental credit card is that you are waived baggage fees. You would reduce that yearly fee from $95 to $65 with one round trip flight where you are checking a bag.
Airline credit cards usually have high interest rates so if you have trouble paying off your credit card month to month then these cards might not be your best option.
After reviewing all the different airline cards I personally decided on the American Express Card. It allows you to earn points with each purchase and then the points can be transferred to most airlines for use in purchasing tickets, upgrades and other types of travel. Additionally, I have family who have used their Amex card this way and have been more then happy with it. Here is an article that has ratings of some of the airline credit cards. I do also have a Jetblue Amex card, but am planning to close that account when it's up for renewal.
What to do when you have lots of points, but nowhere to go or not enough to do anything with? Many programs offer other products you can purchase with expiring points. I've gotten subscriptions to magazines with my old points in the past, a better use then losing the points. You can also donate to charitable organizations, a nice way to give if you don't have the cash to donate.
Here is a list of the current airlines alliances as of Aug 2009:
One World
American, British Airways, Cathay Pacific, Finnair, Iberia, Japanese Airlines, LAN, Malav, Qantas, Royal Jordanian
Sky Team
Aeroflot, Aeromexico, Air France, Alitalia, China Southern, Delta, Continental, CSA Czech Airlines, KLM, Korean, Northwest
Star Alliance
Air Canada, Air China, Air New Zealand, ANA, Asian Airlines, Austrian, BMI, LOT Polish Airlines, Lufthansa, Scandinavian Airlines, Shanghi Airlines, Singapore Airlines, South African Airlines, Spainair, Swiss, TAP Portugal, Thai Airways Int., United, US Airways
In late October Continental will be moving from Sky Team to Star Alliance.
And to give credit where credit is due, I started to understand this whole airline miles stuff when I read Chis Guillebeau's Unconventional Guide to Discount Airfare.
