Motivation doesn't happen on its own. Just like you schedule your runs, you also have to plan to get (and keep) yourself motivated to run. Here are ways to improve your running motivation:
Run with Buddies

Yellow Dog ProductionsWrite It Down

Photo by TripodKeeping a training log is an excellent way to track your progress and stay motivated. It's easy: Just get a notebook and write some notes after each of your workouts. Be sure to mark the date, your approximate mileage and time, and a few comments about how you felt (i.e., "finished strong", "felt sluggish first 2 miles"). Some runners keep a running blog and write in detail about all their runs. Whether you keep a training log or a running blog, looking back at your previous entries will remind you how far you've come and give you more incentive to get out there again.
Find a Mantra

Photo by David MadisonPicking a short phase that you play over and over in your head while running can help you stay focused and centered. It can be your inner motivation when you need it most. Finding a mantra isn't hard: It can pop into your head as you're listening to your iPod, chatting with training partners, or flipping through a running magazine. Pick one that fits your running style and personality, such as: "Easy does it" or "Harder, faster, stronger" or "Never give up".
Also see: Motivational Running Quotes
Talk to Other Runners

Brand X PicturesIf you're feeling unmotivated or nervous about getting started with running, visit the Running & Jogging forum or talk to friends who run and exchange ideas and support for staying motivated. You'll may be surprised to find out that all runners experience periods of laziness when running is a struggle.
Reward Yourself

Pinnacle PicturesEvery now and then, treat yourself to some new
running gear, such as a new shirt, or a massage (this also helps with injury prevention) as a reward for your hard work. The treat will make you feel energized, and reinforce your commitment to your training.
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7 Running EssentialsDon't Think "All or Nothing"

Photo by StockbyteIf you don't have time to do an entire workout, don't take the "all or nothing" approach. If you're really short on time, go for a quick 2-mile run or get in 20 minutes of strength-training. You'll still get some benefits and you'll feel much better mentally for not skipping a workout entirely.
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Can I Split My Long Runs?Remember the Health Benefits

Photo by David MadisonOne of your goals in training for your half or full marathon may be to improve the way you look and feel. So make sure you periodically remind yourself of the health improvements you're making. Get your blood pressure tested, or step on the scale and check your weight. Think about how much more energy you have, and how you now have a healthy way to relieve stress.
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10 Great Reasons to RunCut Yourself Some Slack

Brand New ImagesSometimes you get stuck at work, or you're really tired, or you need to deal with another life issue and you just don't have time to run. It's OK. Don't beat yourself up just because you don't stick to the exact schedule. Your body won't lose its muscle tone or fitness because you wait an extra 24 hours to run again. In fact, sometimes a rest day can benefit you more than a workout. Accept that you will have bad days or missed days, and then get back on track.
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What If I Have to Take a Break from Running?Make Your Goals Prominent

Photo courtesy of goneforaRUN.comPick a goal -- such as a
5K or a
half-marathon -- and tell people about it. Post your
training schedule at home and work, so you have constant reminders about your goals. And celebrate your accomplishments, too. Display medals or photos from your races so your achievements keep you motivated.
Have Fun

Photo by SilverstockThat's really what it's all about. Don't make running another stress in your life. Instead of "no pain, no gain" think "no fun, no run."
More: Get Out of Your Running Rut