Being motivated is sometimes.. Hard. I won't lie, I think about having a drink, or eating a pie, maybe not doing this one work out, just today, just this one time. A lack of motivation can sometimes ruin us, whether it be studying for a test, or working out.. But as everyday, I have some tips for you.
Have a goal: From my own experience, I think I used to get pretty lazy at times because I was trying to do too much in too little time. Set a reasonable goal. You cannot maintain your energy and focus on everything, you'll run out, and fall. Just stick to it completely. Nothing else.
Make a different approach: This one thing might just not be working out for you. Every body is different, every body trains different, you have to find what works out better for you. Although I do not recommend changing workouts before 2-3 months have passed. You can't see a workouts effect in two or three weeks.
Note your progress: While you should be having notes of your progress anyways, you can use them for motivation. Even if you just started out - look at it, you've made the first step. Now, if you give up, you'll have to make the first step again, while you could have been on the second already..
Get excited about your goal: This sounds kind of stupid, right? But you should. Even if at first it would feel a little fake, you have to realise that you're doing something for the better you.
Build up anticipation until the event: Post-poning doesn't sound too good of a tip, but it really works. You set a goal. Do not start today. Start tomorrow, or the day after tomorrow, okay, next week, but not later!
Think about it everyday: You should think about the goal you're trying to achieve everyday, it will stay on your mind, and you will continue wanting more, even if it's a small goal, small steps lead to revolution, or in this case, the better you.
Dream about it: We all dream, why not make dreaming useful? You could dream about you already having accomplished your goal, about the reward you'll get, about all the compliments, maybe some more respect. You should remember not to dream all day though, you need to start with small goals.
Make negative thoughts positive: It's a long ride to the gym.. Well, you're going to be riding, that's some cardio, you're going to go to the gym, that's going to help you even more. You should always try to see the good of things, this applies to life too. You have to study, which is boring and hard? You'll get a degree and money!
These are, in my opinion some good tips which I've been using, I'm sure these can help you get off that chair of yours!