I am very surprised this subject was not already here. Many of us do not have the money to spend $300 to a $1000 plus on a tree. I started growing and experimenting with growing trees about 10 years ago. I (I think we) would love to here how all of you grow your trees. I know this is very broad considering all species, but lets give it a try.
For me I look at the quality of the trunk and nebari first, then a the primary branching while it is in the ground. If you want a thicker trunk, there is only one answer for me; put it in the ground. I use a grow box when I plant mine. It is a 2X6 or a 2X4 box with 1 1/2 in. slots on 2 sides left open to the ground. My boxes are normally 2 ft. square. This makes the tree easy to remove; just unbolt the 2 slotted sides and cut the roots and the tree is out. No digging required.
First thing I do is plant the tree on a ceramic tile to send the roots out from the trunk then while the tree is still thin, get desired movement in the trunk with wire. Trunk taper is controlled with branches and foliage mass. the more foliage the more the trunk will thicken at the branch height and below. Primary branch thickening is controlled in the same way. I have to keep a close eye on the growth of deciduous trees because they can grow rapidly when they are well rooted in. The tree will have to be removed every 2-4 years to control this and root control.
So tell us how you grow trees. I would like to hear how some of you grow pines, junipers and spruces. Here are a few examples of mine.