The best way I have found to stay motivated to train, is to keep track of my stats.
Here are some statistics you can use to measure your progress at the gym:
Pictures of your body.
Videos of you posing or training.
Measurements using a tape measure:
Body weight via scale
Body fat via calipers, Omron, other body fat tests.
Strength – how many reps you can complete:
Endurance – how long it takes you to complete an exercise or event.
Flexibility – record how many inches you can stretch.
Diet – keep a food journal to keep improving your diet.
Test yourself as often as every 2-4 weeks and record the results each time. This will provide you with tangible results for your training, which translates into motivation.
Publish your results online, show them to friends and familly. This will provide additional accountibility to stay disciplined, continue making progress, and ultimately to acheive your goals.
Get a workout partner who is similar to you in body structure, appearance, and goals. Compete with that person to see who can acheive their results the fastest, or to see who can get in shape quickest.