Just because most of your work is done on the computer screen doesn’t mean marking up hard copy is obsolete. Print out a copy of this proofreading cheat sheet and post it near your desk.
The Chicago Manual of Style Online: Proofreading
The Chicago Manual of Style started in the 1890s as a single sheet of typographic fundamentals drawn up by a University of Chicago Press proofreader. Over the years this authoritative style guide has been updated roughly once a decade. The 15th edition was published in 2003, and was heavily revised to reflect the emergence of computer technology and the Internet in publishing.
Source: This tip inspired by the dozens of old, faded, photocopied, proofreading guides we see tacked to the walls and cubicles of designers, copywriters, and e-pro artists across Seattle. This cheat sheet comes from The Chicago Manual of Style (Printed edition is $35 at Amazon).