Reb Noson shares the following advice on how to be successful with his son Reb Yitzchok:
Now, my son, be strong and courageous enough to grab ahold of your thoughts however you can! When it’s time for business, concentrate on doing business. Do everything with joy and without anger, depression, worries or feeling lowly. And when it’s time to pray, force your thoughts to connect to what you are saying and bind them to the words with a tight, mighty bond. Even if you have to pray quickly — just keep your mind from other thoughts. And even if your mind does suddenly steal away and flies off to wherever it does, grab it forcefully like a horse by its halter and return it to the holy words of the prayers. Even more so, when you are studying, keep your thoughts on your studies; and here it should be easier, since after all you must understand what you are studying. Do not be concerned in the least over what else is happening with you right now. The main thing is to stay away from doing bad. We have already spoken extensively about the power of the elder of holiness (Rebbe Nachman) who himself said that, even if he were to commit the greatest possible sin, he would not fall whatsoever. After sinning, he would still be righteous, just as he was before. It is only that afterwards he would repent (Tzaddik #453).
Source: The Collected Letters #433