Avoid vendor up-sell. Settle on a network architecture that meets your business needs for the next two years and try to forgo the extra bells and whistles until you really need them.
Servers with preconfigured OSs save no time. You'll reinstall anyway.
Default cable management isn't acceptable.
Measure elevators and doorways before ordering equipment. You might still need to remove elevator roofs, but at least you won't be surprised.
Don't forget maintenance. Things break, so make sure to figure out what your business exposure is. If you're a 9-to-5 business, maybe 24x7x365 coverage is overkill.
If it appears complex, it will be easy. If it appears simple, it will take forever.
To install or not to install? Installation by the vendor costs $$$; decide up front if you have the chops to do it yourself--it could save you major moola.
Detailed and exhaustive planning from the onset is a pain in the butt, but you'll benefit from it down the road. The corollary is also true: Failure to plan will cost you down the road.
Write it down; if you don't, you'll forget. There are just too many details to track during such an extensive project.