Lessons I've learned from darkrooms I've worked in or put together:

When you decide where you are going to put safelights and regular lights, make sure that you don't end up with a situation where your own shadow falls on the working area.

When you design your area for trays, be sure to include space to empty the trays easily.

Remember to include a dry area for a paper cutter and one or more paper safes/drawers.

Lots of electrical outlets, spread over more than one circuit.

Separate the safelight and regular light switches.

A floor that will withstand spills and is easy to clean.