im very surprised by the question as it is a very central technique I use. mindfulness of thoughts is so very important! the method is called four supreme efforts. EFFORTS!
the buddha recommended many method to substitute unwholesome thoughts. how can one be mindful of his thought any other way but by substituting unwholesome thoughts with wholesome. any unwholesome thoughts that stay in the mind creates suffering. as s0on as one is mindful of his thought and see a unwholesome thoughts, he must do something about it. not observe it, why would you want to observe a thought that creates suffering? you have to substitute the unwholesome by the wholesome. the buddha said that and recommend that.
However, once the unwholesome thought have been replaced by wholesome thoughts, we have to go back to mindfulness of body immediately because its much more calm and conductive to calm and peace.
Ayya khema talks about it in great length and put a HUGE emphasis on the 4 supreme efforts in a lot of her talks.
mindfulness is the entry door. if you dont know what to do when faced with unwholesome thoughts, you must be very serious about this thread and look carefully at the instructions given on this most important matter.
the four supreme efforts are:
(1) not to let an unwholesome thought arise which has not
yet arisen,
(2) not to let an unwholesome thought continue which has already arisen,
(3) to make a wholesome thought arise which has not yet arisen
(4) to make a wholesome thought continue which has already arisen
''
The Buddha gave very exact instructions how to counteract any unskillful mind states and produce skillful ones. They can briefly be expressed as "avoiding," "overcoming," "developing," and "maintaining," and are called the four supreme efforts, which have been briefly mentioned before. They are part of the 37 factors of enlightenment, so must be part of our practice. When perfected they are part of the enlightenment process.
The four supreme efforts are, in the first place, the avoiding of unwholesome, unskillful thought processes. If we look at them as unskillful, we can accept the fact of learning a new skill more easily. Avoiding means we do not let certain thoughts arise, neither reactions to moods, nor to outside triggers. If we find ourselves habitually reacting in the same way to the same kind of situation, we may be forced to avoid such situations, so that we can finally gain the insight which needs to be culled from it. While we are reacting to a situation or mood, we can't assess it dispassionately, because our reactions overpowers the mind. ''
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. Thankfully, we have the basic guidelines laid out, and rest can be experimented in an interactive way. The following article explains this further: