Street Photography Series
How to Focus on a Zone - DOF Scale
Many (in fact all the ones I own) older lenses will have a handy DOF scale or guide built into the focus and aperture rings on the lens. This article has a fantastic shot and explanation of how the guide on your lens can help you. As I've said, you need to have a reasonably good judge of distance to use this method, but if you don't, try it out, because you will soon learn to be able to judge the distances better.
Without a DOF Scale
If you are using a lens which does not have a handy DOF guide (most newer lenses do not) you can still use the zone focusing technique, however you will need to pre-calculate your DOF (use the online calculator here), or use an application such as Photobuddy to help you out (this is the what I do).
Does It Work
I have used this technique in the past with my 50mm lens. I set the aperture to f8.0, then focused to 8m on the focusing scale. This gives a DOF of 11m (5.3m to 16.3m) on my K200D (the sensor crop factor does play a factor here!). Then when shooting all you need to do is pick your subjects within that range.
Good luck with your Zone Focusing!