When developing bonsai, 2 elements are important. The shape and position of the branches and keeping fine branching alive. For both of these aspects springwork is important. In spring trees grow extremely fast for a few weeks. This may cause the canopy to get so dense that weaker branches die off. The branches that do survive shoot straight up without any interesting movement. By correcting the young growth directly in spring, one can use baby bending to get tight curves and fitting movements. Furthermore, by corrective trimming, weaker branches are supported.
Why thin bonsai in spring?
It is good to regularly thin out the canopy. This ensures sufficient light enters the inside of the crown to strengthen weaker inner growth and it encourages back-budding. Equally important though is the enhanced air circulation, reducing the risk of disease. Furthermore, it gives you a better view of any pests that decide to make your tree their home.
Bonsai ulmus before spring trim 
Bonsai ulmus after spring trim
How to thin bonsai in spring
When I see trees getting too dense I start by removing some leaves. This usually is when about 6 leaves have grown at each growing tip, but this depends on the species, Typically I remove the 2 oldest / innermost leaves from the branch. These are deep in the canopy and removing them will directly give you a much better view of the inside of the tree.
Subsequently I verify each branch. In spring a lot of new branchlets have formed, often in the crotches of branches. Normally these are not needed. So every young branch that is not needed is taken out. Also, keep in mind: Mature trees often have very sparse foliage on the inside of the canopy. Once the main structure of the tree has formed, new branches on the inside are removed, unless needed for a cutback point.
Bonsai hawthorn before spring trim Bonsai hawthorn after spring trim
Most plants are stronger on the top than at the base (Apical dominance). This can be compensated at this time of the year by removing more leaves from the upper part of the tree, and trimming back strong shoots from the top. You leave more leaves and do no reduce the lower branches as much. This will make the lower branches relatively more vital, growing more strongly.
Finally I check the whole tree for triplets: Every spot on the tree where more than two branches come together I reduce to twos. On the main trunk this of course means that only one branch can sit at a specific level.
Which branches to remove
Depending on what I would like to achieve I either remove the weaker or the stronger shoot. If I just want to get strong growth to develop the thickness, I might keep individual runners. However, if I am trying to refine the canopy, I remove the strongest shoots and keep the weaker ones. Furthermore I look at the direction of branches. I prefer directional change. So when I can induce directional change (movement) in a branch by pruning the continuous branch out, I will.
Wiring branches in spring
Malus before spring wiring 
Malus after spring wiring
As mentioned; Early spring growth is often straight as an arrow. This takes away the illusion of an old tree. Ideally you want branches with little curves, angles and directional change. Wiring can help bring this into the branches.
Normally branches change direction at the buds: When a side-bud starts growing, the young shoots has a species-specific angle to the main branch. This defines the structure of the canopy and provides the species its own look and feel. When growing bonsai the distance between buds is often too long to get smooth movement in the branches by just relying on direction change at the bud. Wiring the space between buds helps to make smooth movements. By wiring in early spring you can still bend this area and within a few weeks, as soon as the young growth lignifies, the branches set.
