We often use these simple meditation exercises by Sri Chinmoy in our classes.
1) Feel that you are breathing in not air but cosmic energy. Feel that tremendous cosmic energy is entering into you with each breath and that you are going to use it to purify your body, vital, mind and heart. Feel that there is not a single place in your body that is not being occupied by the flow of cosmic energy. It is flowing like a river inside you, washing and purifying your whole being. Then when you start to breathe out, feel that you are breathing out all the rubbish inside you - all your undivine thoughts, obscure ideas and impure actions. Anything inside your system that you call undivine, anything that you do not want to claim as your own, feel that you are exhaling.
2) Each time you breath in, try to feel that you are bringing into your body peace, infinite peace. The opposite of peace is restlessness. When you breathe out, try to feel that you are expelling the restlessness within you and also the restlessness that you see all around you. When you breathe in this way, you will find restlessness leaving you. After practising this for a few times, please try to feel that you are breathing in power from the universe. And when you exhale, feel that all your fear is coming out of your body. After doing this a few times, try to feel that what you are breathing in is joy, infinite joy, and what you are breathing out is sorrow, suffering and melancholy.
3) Keep your eyes half open and imagine the vast sky. In the beginning try to feel that the sky is in front of you; later try to feel that you are as vast as the sky, or that you are the vast sky itself. After a few minutes, please close your eyes and try to see and feel the sky inside your heart. Please feel that you are in the universal Heart, and that inside you is the sky that you meditated upon and identified yourself with. The universal Heart is infinitely vaster than the sky, so you can easily house the sky within yourself.
4) In order to purify your mind, the best thing to do is to feel every day for a few minutes during your meditation that you have no mind. Say to yourself, "I have no mind, I have no mind. What I have is the heart." The after some time feel, "I don't have a heart. What I have is the soul." When you say "I have the soul" at that time you will be flooded with purity. But again you have to go deeper and further by saying not only, "I have the soul," but also "I am the soul." At that time, imagine the most beautiful child you have ever seen, and feel your soul is infinitely more beautiful than that child. The moment you can say and feel "I am the soul," and meditate on this truth, your soul's infinite purity will enter into your heart. Then, from the heart, the infinite purity will enter into your mind. When you can truly feel that you are only the soul, the soul will purify your mind.