1. Do a long long run. 18-25 percent of your goal weekly mileage. It doesn't have to to fast. You can walk part of it if you get tired. You just have to finish it. If you run five or less times a week and are training for a marathon you can have a long run up to 30 percent of your weekly mileage.
2. Take easy days. Of course this also implies harder or longer days. Try not to run the same distance two days in a row.
3. Run slower. If your goal is just to cover a distance take it 10 seconds a mile slower, or more.
4. Have some goal (besides the mileage). Trying to qualify for the Boston Marathon or run a PR at whatever distance will help keep you motivated on the cold, wet, windy days.
5. Stretch. I prefer active stretches like swinging my legs before runs and static stretches after my runs. A flexible runner takes less energy to move his or her legs.
6. Run with friends who are trying to do the same amount as you are. If someone else is suffering doing the same thing it makes it more bearable.
7. Run a different route every day. This keeps it mentally new and interesting.
8. Keep a running log. This way you can track your progress and see accomplishment. I use Running Ahead.
9. Do a medium long run. This is kind of a more advanced method but it is good for aerobic development.
10. Run twice in one day. This can take some getting used to but it is a sure fire way to run more miles in one day with less stress on your body than the same distance in one run.
11. Celebrate the little things. Your first 13 mile run! Your first 70 mile week! A 5k PR by only 2 seconds! Keeping a positive attitude keeps you working.
12. Don't be a slave to your mileage. If you are hurt, injured, burntout, tired, or sick take the day off. A day or even a week off now is better than three months off down the road because you didn't take care of yourself.
So those are some steps you can take in order to run more miles. Get out more!