This week I delivered five electrical prototypes to a world renowned industrial designer. Here's a few lessons I learned:
Get the mechanicals as soon as you can. I have yet to see a product where the first mechanical and electrical prototypes fit together perfectly. Have the mechanical guy there when you first assemble them together to discuss any problems right away.
Mechanical guys like datasheets but they love datasheets plus samples. Give them samples of everything, particularly switches, connectors, displays, light pipes, etc.
Better yet, have the mechanical guy pick out the switches, connectors, displays, etc. based on the requirements you give them.
Good things to have on-hand for modifications to the enclosures: Dremel, hot-glue gun, and strong double-sided tape.
For tight designs, some of the higher-end PCB layout programs (Altium, for example) will produce a 3D model of the PCB from the layout. This is very useful, and then creates a 3D space contract between electrical and mechanical. But be sure that your heights are all correct! Even better, have someone else double-check your design.
When determining how much space you'll need, don't forget to take into account space for wiring etc.
For a prototype, try to get as much space as you can. There will always be surprises.
For components that are very tall, try to find lower height components, even just as a backup. One of the prototypes I just delivered had a speaker that was about 2" in diameter and 1" tall. The mechanical guy screwed up and the space for it was nowhere near large enough, a good 0.2" too short. But, I found a couple other speakers that were similar but thinner, and I swapped one of those in and looked like the hero.
Speaking of Heroes and Villains, at some point both you (the electrical guy) and the mechanical guy will screw up something, so don't be too hard on each other. Be nice when they screw up and chances are they'll be nice to you when you screw up.
Spare, Spares, Spares: if you're working on a product for a demo, get extra enclosures and make extras of everything. On one of my prototypes a stupid little $0.20 battery holder prevented the a system of 3 devices from working. Of course it worked fine in the lab and failed when I got over to the client. Unfortunately I didn't have any spares so I had to troubleshoot it and fix it there on the spot. Doh!