When developing a bonsai you will find that the top of the tree tends to grow faster than the bottom. this is called 'Apical dominance'. You will never completely overcome this. In the early stages of the trees development you will need to prune the top of the tree harder than the lower branches.
Differential pruning overcomes the trees natural tendency toward 'Apical dominance', that is the tendency for most of the growth to happen at the top of the tree. This is undesireable in a bonsai.
By removing more buds on branches at the top of the tree, than those lower down, you will see a better structure develop in the lower branches, they will thicken, as will the trunk lower down the tree, giving a better trunk taper.
I tend to do this to my deciduous trees in the early spring, prior to bud break, then again if the tree is leaf trimmed.
Conifers are treated in the same way, except that the buds are plucked off, rather than cut.
There are two things to lookout for when pruning your trees, things which you will not see on mature trees.
You should try to remove any foliage hanging down from the branches, allowing the branch structure to be seen from the side.
You should remove any foliage growing at the point where the branch and trunk join. This happens mainly in conifers.
© Allen. C. Roffey 13:30 12/07/98