The best time to plant a tree is 20 years ago

The next best time to plant a tree is today!  One of my norway maples died, and after wrestling with a hand saw to cut it down, I figured it was a perfect time to plant something a whole lot more fun and interesting.  Buying a tree in August means there is not a lot of selection left.  But I was happily surprized to find a bunch of Sunburst Thornless Honeylocust (Gleditsia triacanthos var. inermis 'Suncole').  These trees are quite graceful, and their branches droom artfully.  
Being August, the trees were not in the best shape, and with much hemming and hawing, I picked the least damaged specimine.  We stuffed the pot between the front seats, and the top branches hung out the back of the car.  Covered in plastic to reduce wear and tear, we got it home. 
Water well while still in the pot, so that the water leaks out the bottom.  Cut off any bushy growth at the base of the tree or anywhere you don't want a branch to form.  
Picking a location for a new tree isn't easy, because you know it will be something to look at for many years to come.  I picked a spot beside the stump of the old tree.  I marked out a circle about 3 feet in diameter, stripped off the soil, and started to dig.  With so little rain this year, the soil was like concrete.  The digging was a slow process.   
The hole should be as deep as the pot, and at least twice as wide.  Set the tree in the hole and lay a shovel across to determine if the depth is level with the base of the tree.  Then loosen the soil in the bottom of the hole, fill it half full of water, and let it drain.  Gently pop the roots out of the pot (this might take some doing), tuck the tree into the hole, and if it was pot bound, loosen the roots.  Set the tree so its best side is facing the way it will be seen.  Water the hole again, put half the soil back in the hole, tamp it down, water again, and add more soil.  Don't amend the soil with good stuff, because the roots will not want to travel away from the rich soil, and the tree won't become well established. 
Top off the planting with mulch to keep the soil moist.  Support the tree for the first year.  After I got it planted, I noticed that I missed trimming a dead branch.  Now that it's in the ground, it's too high to reach.  Since it has been so dry here, I make sure to give the tree a gallon of water every day.  It looks pretty scraggly, but I'm pleased with the result.