Once you have a candidate design, go around your kitchen and make a plan of everything you currently have in there and work out where you will put it. Make sure there is a place for everything. Make sure you have somewhere to put tall stuff like bottles and cereal packets - that one nearly caught me out.
Have you got any tall appliances? Make sure the wall cabinets are set high enough to fit them underneath.
Have you got big dinner plates? If you want to store them in wall cupboards, make sure they are deep enough.
If you have corner cupboards and it is at all possible, go for L shaped ones and not ones with a pull out mechanism. The L shaped ones hold so much more stuff.
If you have corner cupboards and L shaped ones won't fit, then definitely do get pull out units, otherwise stretching into the blind corners will drive you mad.
I went for the biggest single sink I could fit into our 600mm base unit, rather than a 1.5 bowl, it is massive and I absolutely love it - though this will depend on whether you do much washing up by hand, I think.
Think about lighting. Make sure you have lights under the wall cabinets to light the worksurface and stop you always working in your own shadow. Also, put extra ceiling lights in directly over any worksurface that doesn't have cabinets over it. I wish we had got a few more spots in over the sink.