Kitchen cabinets are all about planning, planning, and more planning. I start with a detailed drawing showing the existing locations of the plumbing, electrical, and openings (windows, doors, etc) in all walls. Working with the cutomer, I then determine what can and can't be moved (i.e. sink drain lines, stove/oven gas or electric, window/door openings, etc). At this point, I tell the customer that they need to commit to exact models of their sink(s), and appliances (stoves, ovens, range hood, ice maker, fridge, etc).
I do a first draft design around the "fixed" cabs, then open the floor to whatever they want for the remaining space. Most wives absolutely love being able to decide on things like drawer sizes, amenities like pull out trash drawers, spice racks, cookie sheet storage, etc.
Notice that I haven't thought about materials, etc? I'm all about form following function and want the function nailed down before I get into things like appearance.
I completely disagree with wes's statement about Sketchup. It can be useful, but a kitchen can be designed lots of ways - including paper and pencil. I use a full-on 3D CAD program (TurboCad), but you probably don't want to go there. The learning curve can be pretty steep and long.