1. Define what your goal is
Come up with a specific definition of what your goal is. Don't make it something vague and generic, make it something very specific. Your goal might be to be successful: but successful in what? If you tell yourself you just want to be successful, you're not giving yourself anything specific to aim towards. Tell yourself you want to be successful in a particular field - painting, for example - and you've given yourself something that you can work towards.
2. Be realistic in your approach
You aren't going to achieve your goal if you sit back and just wait for it to happen; you have to actively make it happen and to do that, you have to have a realistic approach. It's not necessarily going to be easy all of the time and you will surely encounter many stumbling blocks and pitfalls along the way. In other words, it's not going to be plain sailing, so don't kid yourself into thinking it will be. Have a realistic approach to achieving your goal: remind yourself that achieving it will sometimes be hard and that it's probably going to require a lot of time and effort on your part. When you have a realistic approach, you have a clear idea of what exactly you need to do to achieve your goal.
3. Use positive and negative thoughts to your advantage
Think of how good you will feel when you achieve your goal. Imagine how excited, happy, satisfied and proud you will feel once you've finally done what you've wanted to do for so long. Use these positive feelings to encourage you to do something towards achieving your goal. You're not going to experience these feelings unless you achieve your goals, after all. Likewise, there are negative feelings associated with not achieving your goal, such as disappointment, upset, lack of self-identity and self-worth. Think about these negative feelings and use them as motivation to work towards your goal. Negative feelings such as the ones mentioned are feelings you don't want to experience, but you will experience them unless you do something about achieving your goal.
4. Don't allow yourself to get sidetracked
If you really want to achieve your goal, you shouldn't let things get in your way. Of course, you've got to get on with real life, but don't let that deter you from what you're aiming towards. Keep focused on what it is you want to do and do a little something every day: it's much better to keep the ball rolling and to keep momentum going. Doing a little something every day is a good way of keeping your goal fresh in your mind. You could also try setting yourself daily reminders of your goals. Think of something you see every day - it could be your bathroom mirror, for example. Stick a small photo of something related to your goal on your bathroom mirror and there's a daily reminder right there.