I had a cardboard mockup of our panel sitting over our couch in the little-used living room for a couple of months. Every time I'd go by it, I'd admire it. I was totally in love.
Then, mere hours before I was about to start cutting metal, a friend popped in for an unannounced visit. He very casually asked "why not move the engine instruments and place them between the EFIS and moving map GPS?" I quickly realized the point he was trying to make, every so subtly. The bezel of the GPS would have partially obscured the view of the left side of the steam gauge engine instruments.
In a matter of minutes I had repositioned all the paper instrument mock-ups and found this new layout was MUCH better than the design I had come up with. The next morning I started cutting metal using this new layout. Eight years later I'm still happy happy happy with the layout.
While a prototype is a good idea, it's sometimes not fully practical, although 3-d models made from cereal box cardboard work well. As my experience taught me, having another set of eyes, or a few sets of eyes, look over your design will often find a flaw that we overlooked.
As sexist as it may sound, if you are a male (as so many on this forum are), get a female to give her feedback on the panel design. I've had some really good alternatives suggested by females, alternatives that no male had come up with. It's that 'left brain right brain' thing, I guess. My wife and daughter have both sat in front of the panel for our new airplane. Neither are pilots. Both have come up with ways to improve the design, and in doing so, they've made that little part of the airplane theirs. Win-win!