Use boards on edge and not laying on the face (unless using 4"x4" or other square cut). For example, I would use a 2x4 like this || and NOT like this = .
Overbuild it. It's easy to overbuild something to hold a 400 gallon tank and have peace of mind and less headaches down the road. When in doubt, go big!
Measure twice and cut once.
Plan it like water will be touching everything and getting it wet. It will happen eventually through no fault of your own. Devise a strategy to make everything accessible without having to pull out a 55 gallon sump.
Build it so you DON'T have to take the stand apart to get the sump in/out. It will happen eventually.
Plan for EVERYTHING to go wrong.
Keep a box of bandaids/6-er of cold brews/Tylenol handy.
Inspect all wood at store and ensure that it is straight (look down the egde).
OK, that's it for me.
Dana