An excerpt of two exercises from Franz Bardon's book: Initiation Into Hermetics.
1. Take a seat in a comfortable chair or lie down on a settee. Relax the whole body, close your eyes and observe the train of your thoughts for five minutes, trying to retain it. At first, you will find that there are rushing up to you thoughts concerning everyday affairs, professional worries, and the like. Take the behaviour of a silent observer toward these trains of thoughts, freely and independently. According to the mentality and the mental situation you happen to be in at the moment, this exercise will be more or less easy for you. The main point is not to forget yourself, not to lose the train of thoughts, but to pursue it attentively. Beware of falling asleep while doing this exercise. If you begin to feel tired, stop instantly and postpone the exercise to another time, when you intend not to give in to tiredness. The Indians sprinkle cold water on their faces or rub down the face and upper part of their bodies to remain brisk and not waste precious time. Some deep breathing before you begin will also prevent tiredness and sleepiness. As time goes on, each disciple will find out such little tricks by himself. This exercise of controlling thoughts has to be undertaken in the morning and at night. It is to be extended each day by one minute to allow the train of thoughts to be pursued and controlled without the slightest digression for a time of 10 minutes at least after a week's training.
2. Let us then learn how to produce an absolute vacancy of mind. Lie down comfortably on a bed or sofa or sit in an armchair and relax your whole body. Close your eyes. Energetically dismiss any thought coming upon you. Nothing at all is allowed to happen in your mind; an absolute vacancy of mind must reign. Now hold on to this stage of vacancy without digressing or forgetting. At first, you will manage to do so for only a few seconds, but by practicing it more often, you will surely succeed better at it. The purpose of the exercise will be attained if you succeed in remaining in this state for a full 10 minutes without losing your self- control or even falling asleep.
An excerpt from Swami Sivananda's article: Various Methods of Mind-Control.
Sit alone and watch the Vrittis (thought-waves) of the mind. Be indifferent. Remain as a Sakshi (witness). Don't identify yourself with the Vrittis. The mind will then be under your control.