The first step is working out why you want to change? For example, you might want to be healthier. But why? To loose weight? Maybe, but why now? To look good for a holiday or to fit in that dress for that event. Whatever it is, hold on to that. Remembering why is always key to staying motivated.
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Be Patient
Don't beat yourself up because you haven't seen results as quickly as you wanted or because you took a step back, it doesn't matter. What's important is that you don't give up, that you're still trying. You need to remember to praise yourself over the little victories. If you wanted to loose a pound this week but stayed the same weight don't worry because you didn't put on a pound or even if you did it doesn't matter, just work twice as hard next week.
Routine
Make a routine. Having a routine makes it easier to stay motivated because you know what you're meant to be doing at every point of the day, making it easier to stay on track.
Treat yourself
Last but not least, reward yourself. Rewarding yourself is possibly the most important part of the whole process because this is what keeps you going the most. Always thinking of that treat you'll have. The rewards and goals are completely up to you, just remember you can't have those rewards if you don't keep going.