Hillel the Elder said some good things. For some reason, this one meant a lot to me, especially the part I italicized.

In Avot, Hillel stated "If I am not for myself, who is for me? and when I am for myself, what am I? and if not now, when?" The third part contains the admonition to postpone no duty, the same admonition which he gave with reference to study (Avot 2:4): "Say not, 'When I have time I shall study'; for you may perhaps never have any leisure."

One of the most important things to me is keeping my word. If I say something, I'm going to do it, come hell or high water, even if it costs me personally. Hillel's quote can also be a helpful motivator; if you start to put things off, you'll keep putting them off and either not ever get them done, or do them in a rushed and poor fashion. I'm horribly guilty of the latter, repeatedly, but hopefully I can remember the words of the sage Hillel and improve myself.

Quoted from Wikipedia.