This article was originally only for students who needed motivation for study and in the "Ideas for Teens" section.  However, given that it’s not just teenagers who have difficulty motivating themselves to do difficult things, I thought I would reproduce it here, with a few extra ideas for adults.
 

1. As soon as you get home (or get into your free study period), get all your books out, open and ready so that they are easy to see and get started with. Even if you go and have a break before you actually start, make the first thing you do to get ready for study. The easier it is to start, the more likely you will start.

For adults: make the “first step” you have to take to do something difficult, as easy to do as possible.  Have your gym gear out next to your bed, keep the broom close to the kitchen – and so on!

2. If you are someone who does homework for the subjects you like, and never for the subjects you don’t, then do this: Start with something OTHER than the subjects you like. Tell yourself you are going to do JUST ten minutes of this other subject. Whatever happens, you will MAKE yourself do ten minutes and then you can switch to the subject you like.

For adults: once you have started doing something unpleasant, tell yourself you will make the first ten minutes the hardest.  Throw yourself into it and get the worst bit done.

3. Use the clock to help motivate yourself to study in SHORT blocks. For example, say to yourself, okay I will start this English essay and just do 15 minutes worth before I am allowed to stop. Keep an eye on the time and coach yourself through it every time you get distracted – “okay, only 11 minutes to go and then I can have a break … okay 7 minutes to go and then I can stop this stupid essay … come on three more minutes, keep going”. If you are “on a roll” and want to keep going after the 15 minutes are up, by all means keep going (the only exception to this is with Maths which is too tempting just to spend way too much time on for you), but make a deal that you must do at least 15 minutes on it.

For adults:  similarly, use a watch to motivate yourself and set yourself short time goals.

4. Use short blocks of time 15 minutes here and there, instead of long blocks of time on things you really don’t want to do.

For adults:  don’t put whole days or weekends aside to do things.  Long periods of time are often not helpful in motivating ourselves.  Do five minutes after tea, rather than wait to the weekend.  Do ten minutes before lunch rather than waiting to get to the three hours put aside for the task after lunch.

5. Use the “on/off” method of listening to music, using it as a reinforcer. In other words, listen to the radio for 15 minutes and then turn it off for 15 minutes, and then back on etc. You can make a deal with yourself that you HAVE to do ten more minutes for example of Physics before you can listen to the radio or something like that. Changing the atmosphere slightly like this will help increase motivation just a bit.

For adults, if you haven't tried listening to music whilst you do difficult things, give it a go.  Especially for tasks which are not too complex.  Listening to something while you work/exercise/do boring tasks can make it a whole lot more enjoyable.

6. Have “focus” days in which you really work hard on one subject in particular. Mondays might be Physics, Tuesdays might be Chemistry etc.

For adults:  find “systems” for times/places you do things, so you are less likely to put them off.  Perhaps Mondays is the day you do the hand washing.  Wednesdays is accounts day.  Saturdays is the day you eat vegetables at every meal.

7. Use rewards. Draw up a whole heap of counters and get a jar/container to put them in. Give yourself one counter after every half an hour of good solid work. Make it that once you have got X numbers of counters then you can get a CD.

For adults:  use actual money!  Or find other rewards.  Tangible, visual reminders of why we are doing difficult things can have a powerful effect on motivation, even if we know why we are supposed to be doing the difficult thing.

8. Keep your medium term goals in sight. Set a goal/s and write them up around your room and in your diary. Look at them often.

9. Work with short term "grade" goals. Every time you have a test/essay/assignment to hand in, set yourself a specific goal for the grade you want to get for this individual test/essay/assignment.  Write it down. Keep it around you to help motivate you. 

For adults:   tell other people about some things you want to achieve and ask them to ask you about them.

10. Also work with short term “task” goals. Set a goal for how much you will complete in the next 15 minutes. Keep setting little goals all the time, rather than just one large goal only once.

11. Understand the horse and cart of motivation: Often motivation follows action. Do not wait until you feel like doing something before you do it. Just start. Motivation will often follow.

12. Vary your "task" habits to keep your interest high. Do things in different ways and in a different order. Be creative.

13. Plan time for things you’d rather be doing – then learn to wait! Write down what you would rather do and plan a time to do it. Also train yourself to wait – for 30 seconds, then 60 seconds, then longer and longer before you stop.