A handwritten answer from the Rebbe to an individual who asked for advice on how to combat laziness:
מהעצות לעצלות – התבוננות לעתים תכופות בתוכן רפמ”א [=ריש פרק מ”א] בתניא ואגה”ק ר”ס כ”א, ובהענין דהזמן אבידה שאינה חוזרת.
Some of the solutions on how to deal with laziness are:
To frequently contemplate on the content of the beginning of chapter 41 of Tanya [which discusses how Hashem constantly scrutinizes each individual and how one, therefore, must constantly fear Hashem and serve as a dedicated servant],
and of Iggeres HaKodesh [fourth part of Tanya] beginning of Epistle 21 [which discusses the importance of the trait of zerizus, alacrity, in service of Hashem].
You should contemplate as well the concept that time that passes is a loss that can never return. ■