Well, one way to earn miles is to fly. If you have heavy travel ahead, it might be a good idea to aim for Elite status. The Elite bonuses add up. If need be, consider a status challenge or mileage runs.

Also, keep an eye on promos and such. If miles is your ultimate goal, you can choose hotels, car rental companies, etc based on which one earns you most miles. (Sometimes it's more lucrative to accumulate points and later convert them to miles.... YMMV)