Whist putting together a presentation for the London Photography Meetup Group (LPMG) I researched into the technique of focusing on the Hyperfocal Distance to ensure maximum depth of field.
The formula for calculating the point at which to focus at a given aperture, focal length is shown below.
Most photographers that I know however would rather not have a maths lesson whilst out and about with their cameras and therefore don't expect to apply this in the field.
A useful rule of thumb is to focus one third of the way into a photo. This will give a reasonable approximation to the actual Hyperfocal Distance required.
There are also a number of Apps available for the Apple iPhone which make calculating the value a little easier. Photocalc by Adair Systems being on of the better ones available IMO