The Stand

Setting the tank on its stand. You'll be cautioned by everyone to make sure the stand is sturdy and level, that it doesn't rock. Well duh, right? But in levelling the tank, don't trust your eye or even a carpenter's level. Instead, pour a splash of water into the tank, only enough scarcely to cover the bottom glass with an even film. You'll see in a flash whether one corner of your stand needs to be shimmed up to level it. Shim the base of the stand, by the way, not the tank!
 
My old apartment is not like your place: no angles here are square, all my floors slope, and water levels in aquariums can be distractingly aslant, with light spilling from one upper corner. No matter how level the stand, you'll want to lay some compressible cushioning under the tank, to make sure that the pressure is equal all round the base. One piece of gravel or a hardened drip of paint can shift all a water-filled tank's immense weight to three points. The tank may develop a leak at a strain point. To avoid this, I always lay strips of felt window insulation all round, directly underneath the tank sides. Now it comes with its own sticky backing. I set the tank down directly on the felt, careful not to displace the strips.
 
At a hardware store you can get strips of black ribbed hard rubber matting that comes in a roll and cuts with sturdy shears to fit the top of the stand. It has just enough "give" to it and it's waterproof.
 
You could also use a styrofoam sheet that you cut to size; the edges can be blackened with waterproof marker. The sheet doesn't have to be much thicker than the styrofoam of a take-out coffee cup, just enough to support its own weight with visibly sagging. If your stand is the old-fashioned openwork angle-iron type, a stiff styrofoam sheet under the tank will also insulate the gravel against heat loss. The heater racks up the kilowatt hours faster than any other aquarium device you're using.