Mark Boulton has started an excellent series of articles on designing grid systems: Five simple steps to designing grid systems
The grid is a regulatory system which pre-empts the basic formal decisions in the design process. it’s preconditions help in the structuring, division and ordering or content. I’m not saying a well designed grid will solve all of your compositional problems, far from it, but it goes some way in creating a coherent structure in design which in turn creates the aesthetic values all of us are after in our designs.
(via design Principles)