Most people who begin a course of self improvement have some kind of emotional trigger -- often negative.
Maybe they feel disrespected, have experienced rejection(s), been dumped, or generally feel lonely or depressed. But there's that stage where they turn that negative emotion into an inspiration and say, "Enough is enough; it's time to change!"
This motivation usually lasts as long as the emotional trigger. More often than not, people start out on fire, eating healthily, hit the gym, hit the books, chat to more women and so on. Every time they do it, they feel the drive and face it head on.
The problem is that when the emotional trigger starts to subside, they start to slack off.
Their diet starts to allow the occasional McDonalds or binge-drinking session. Their gym routine becomes a once in a while visit and they're back in their rut -- remote control and a beer in their hands.
That is, until the next time they feel an emotional trigger, then they start up again.
The key is to keep that emotional trigger fresh and reflect on it regularly. Write it down in detail if you want to and read it every day. Every time you are debating whether to go to the gym or sit and play Playstation, or every time you think of ordering a pizza, remind yourself how you felt when you made that decision to improve yourself and why you should avoid these bad habits.
Personally I made some exceptional physical and mental improvements during the past years and I admit that many of my emotional triggers related to girls. However, I've been in a relationship the past 9 months, and though things are great between us, I ended up getting a little complacent with myself.
I had been a dedicated health freak the past 2 years, and yet in those months I stopped working out so much and started eating snack food. Then a couple of weeks ago my friend made an innocent comment about my "love handles" and it hit me how much I've let myself go! I felt ashamed of myself and felt like I'd thrown away years of hard work.
On reflection I realized that my original emotional trigger had gone.
Being in a happy relationship, I didn't need to attract or impress women or avoid rejection anymore. What I needed was another emotional trigger. And I found one in my disgust at myself at becoming so lazy and I vowed to snap out of it.
Luckily I caught myself in the early stages of complacency. Now I'm back on form, working out hard and improving myself again. This time to keep that feeling of shame at bay.
In short, don't lose sight of your emotional trigger. Use it to help you beat complacency and continue to improve!