Purifying The Mind

Make sure that you don't allow your mind to become chometz -- leavened. Don't dwell on bad thoughts or desires at all. These thoughts are rooted in the side of death. If they come into your mind, just reject them and push them out completely, because ultimately they ruin the mind and make it impossible to pray properly and experience genuine joy. You should try and avoid even the merest hint of thoughts like this. You should be as careful about it as we are to avoid even the merest speck of chometz on Pesach. Evil thoughts are the leaven in the dough, and the law regarding leaven on Pesach is that we must destroy it completely so that it can neither be seen nor found. Purify your mind and empty it of any thoughts like this.

(Rebbe Nachman of Breslov)