Engage in one kind of ...

Engage in one kind of business only, and stick to it faithfully until you succeed, or until you conclude to abandon it. A constant hammering on one nail will generally drive it home at last, so that it can be clinched.
 P. T. Barnum

Quotes from the same author

Every man should make his son or daughter learn some useful trade or profession, so that in these days of changing fortunes of being rich today and poor tomorrow they may have something tangible to fall back upon. This provision might save many persons from misery, who by some unexpected turn of fortune have lost all their means.
 P. T. Barnum
Whatever you do, do it with all your might. Work at it, early and late, in season and out of season, not leaving a stone unturned, and never deferring for a single hour that which can be done just as well now.
 P. T. Barnum
The possession of a perfect knowledge of your business is an absolute necessity in order to insure success.
 P. T. Barnum
Witchcraft is one of the most baseless, absurd, disgusting and silly of all the humbugs.
 P. T. Barnum
Money-getters are the benefactors of our race. To them ... are we indebted for our institutions of learning, and of art, our academies, colleges and churches.
 P. T. Barnum