I find that the best way is to figure out what makes you tick. Some people are motivated by certain goals such as how many calories are burned, setting a time record, distance, or a reward afterward, etc. Me? Just the mention of a gym is enough to get me to crawl under my sheets with some chocolate ice cream and a video game controller. I have to be outside and have a challenge. I live in the backwoods close to the mountains where the forests are great for running. Hills, rocks, and trees create a great obstical course for me. That makes it fun. Find that thing, that way that transforms a workout into something desireable for you. Make it fun!
THAT . . Is tough.
I can identify to a point. Once I am at the Gym, I am all business, but getting there is the first part that can prove difficult. Select which days you will be committing to. Then work a schedule AROUND those days so you have less excuses for not being able to go. Do not sabotage you scheduled day . . Don't eat too heavy, or create a sickness or tiredness to get out of it . . .
If the actual "working out" part is difficult, perhaps try an "In-House" trainer to keep you motivated.
Make small goals, and look at yourself in the mirror . . . Force yourself to actually see the little changes that make it worth while and understand just by going and exerting yourself, you are lengthening your life span.
Make it exciting by having a different, new workout to follow everyday so you don't bore yourself. And perhaps by joining a website or something, that you have to pay for, you'll feel like you have to workout because you gotta get your money's worth!!! That's what i do lol