DEALING WITH OBSTACLES
When you encounter an obstacle, take a little time out to reconnect with your main goal and your reasons for wanting to achieve it. Remember why you're exercising in the first place. How could you still achieve your goal, and find ways around this new obstacle? Your journal can be your friend here - use it to reflect back over why something may have ended up being more difficult than expected, and to brainstorm ideas for different ways to move forward. You can also use it to proactively motivate yourself before obstacles appear - to track your progress in whatever area you'd like to see yourself improving.
GET OTHERS INVOLVED
If you haven't already, create an exercise routine that gets friends or family involved. Studies have shown that people whose friends or family support their goals are far more likely to achieve them, and it's hard not to feel supported when people you care about are working out alongside you. Not only that, but if you've agreed to meet your 'workout buddy' for an exercise session, you make it that much harder to let them down by deciding at the last minute you just can't be bothered today.
MAKE DEALS WITH YOURSELF
If you're having trouble summoning the motivation to get out the door, promise yourself you don't have to finish your workout if you don't want to. Decide that you only have to complete the first ten minutes, and then, if you really don't feel like keeping at it, give yourself permission to pack up and go home again. Most of the time, just getting out the door and starting will be enough.
For a short term motivational quick-fix, don't underestimate the power of bribery! Identify three or four different little things that bring you pleasure, and promise yourself you'll take time out and indulge in at least one of them if you complete your workout as scheduled.
GET HELP!
Finally, if you're noticing increasing difficulty motivating yourself despite trying all of the above suggestions, consider talking to a personal trainer near you. A big part of qualifying as a trainer involves not just learning specific exercises, but learning how to keep clients motivated. If cost is an issue for you, think seriously about hiring an online trainer.