http://blackbumps.com/razor-bumps-prevention-how-to-never-get-them-in-the-first-place/
Razor Bumps prevention Shaving in the direction of hair growth 'along the grain' and NOT against it. This is very important in preventing razor bumps and irritation.
http://www.blackbumps.com
One of the most important things to consider when shaving is to ensure to shave in the direction of hair growth [along the grain], and not in the opposite direction. Each person is an individual and has a different pattern of hair growth, so it is advised to analyse your own hair growth pattern. Shaving ‘against the grain’ will increase the chances of both types of ingrown hairs and may also cause skin irritation. Mastering this technique, especially when using a razor, is key to preventing bumps.
Even with a clipper, I ensure to shave in the direction of beard growth.