First, keep a record of your accomplishments and review them on a regular basis.  If you run, you can keep track of your distance and your pace.  If you strength train, log how many reps you're doing at what weight.  Knowing how much you've gained in terms of working out can help keep you going when you just want to quit.

 

Second, keep a record of your body weight and measurements.  While it's true that working out is not the sole contributor, it can be quite motivating to know how much weight and how many inches you've lost.  

 

Third, keep the fat pants out.  There is nothing better than putting on a pair of pants that used to be too tight, to find that they now fall to your feet effortlessly.  Picturing how horrible you used to feel at that prior size can be just the tipping point you need to get back on track with your exercise.

 

Fourth, keep changing your workouts.  Becoming stagnate in any area of life is never a good thing, and when it happens in your workout, you'll be more likely to discard your sweat sessions.  Choose interesting activities that keep you motivated.  Choose goals that keep you working towards something.  Examples include:  training for any kind of race, trying one new activity a week, or trying to break your personal record.

 

If these tips can't keep you motivated to keep going on, then you should reflect on why you wanted to get healthy in the first place.  When you know how to stay motivated to keep working out, you can power through any internal struggle you may have to stick with it.