Economical writing by Deirdre McClosky is an excellent book of 35 rules for clear prose. In the final chapter she quotes Mark Twain’s seven rules for good style.
The writer should
say what he is proposing to say, not merely come near it;
use the right word, not its second cousin;
eschew surplusage;
not omit necessary details;
avoid slovenliness of form;
use good grammar; and
employ a simple and straightforward style.
These seven are worth following, as are McClosky’s thirty-five.