· Car rental agencies have their own programs, but they will often also give you frequent flier points as a bonus for renting with them.
· Credit card companies are probably the best way to collect additional miles; many cards will allow you to collect one mile per dollar spent on the card. If you travel for business this can add up to a lot of miles. I have averaged 50,000 spending per year in the last few years which is enough for 2 short haul flights in North America or one flight to Europe ever year. Even if you do not travel for business if you put most of your expenses on credit cards it will add up to a lot.
· There is a third category which will also give you miles but in actual fact you are probably better off signing up to their own programs and that is hotels. Hotels usually have points programs of their own which can be very useful. Starwood hotels has a very good program called preferred guest which allows you to collect points to use for hotel rooms, but it has one other special feature- these points can also be converted to points on most major airlines- so if you are missing some points for your next reward ticket you can transfer it from your preferred guest account.
If you get a credit card which is partnered with Starwood preferred guest you have the best of all worlds- you will collect a lot of points for hotels which can be transferred to airlines if needed. In Canada we have the MBNA MasterCard and in the United States there is the American Express for this.