“People need to realize how powerful the transformation of soil can be.” ― Ron Finley
Anyone who has seen the rejuvenation of a bonsai in bad soil knows how powerful the effects of fresh, good, coarse soil can have on a weak tree. I have several trees that have been aching since last summer to shed their lousy poorly-draining nursery soil and get into some of the good stuff. So I'm repotting like crazy.

This poor tree not only needed new soil, it needed to get rid of an ant colony!
If you haven't started repotting yet, it is time. Here in Southern California, repotting can begin much earlier than in other parts of the country, but it's getting to be repotting time in most places now. If you have many trees to repot, prioritize which ones to do first so that you don't miss your time window on any of your trees. I did my pines first. Then I started with any tree with swelling buds. Once they break, the tree will experience more stress from repotting. Once I have those trees done, I move on to flowering species, then non-flowering deciduous trees, then broadlef evergreens, conifers, junipers, generally in that order.
You can bareroot some species, but for conifers, junipers and pines, it is safer to replace the soil over two or three repotting periods. In most other regards, I follow the same basic steps for almost all of my trees.
1. Pre-mix the soil. I don't want to have my tree sitting out with the roots exposed while I mix my bonsai soil.
2. Prepare the new pot. Wire screens into place to cover the drainage holes. If the tree is going to need to be secured in the pot with wire, I place those wires into the pot through the drainage holes and set the pot aside. If reusing the same pot, I do this step just after the tree is removed from the pot, but before any root trimming.
3. Remove the tree from the existing pot. I start by clipping any wires that hold it in. When it's in a nursery container, I usually just hold the tree and smack the sides a few times and try to pull it out. If necessary, I cut roots extending from the drainage holes. If it's in a ceramic pot, I use a root knife or something similar to scrape around the edge of the pot. Once it is loose, I carefully tip or pull it out of the pot.
4. Scrape the root ball with a claw or a root hook. I prefer three-pronged rakes. I first rake away rootless soil on the top, then I comb the roots out thoroughly, especially if I'm going to bare-root it.
5. Trim the roots. If this is a first repotting and the roots are thick and not easily raked, I use a saw to cut away one-third to one-half of the root mass. I use root clippers (not the sharp scissors I use for foliage) to prune back any long stringy roots.
6. Scrape out old soil sitting right under the trunk. I leave a slightly concaved void under the trunk so that later I can mound up the fresh soil and cleanly place the tree into the new pot.
7. Gently mist the root ball to keep it moist, or, if bare-rooting the tree, take it to the hose and hose away all the old soil completely.
8. Put in the first layer of soil for the new pot. I start with larger, courser material, like pumice. Then pour in some of my premixed soil to a level that will leave the new root mass at a pleasing height within the pot. I mound it up a little in the center to fit the concave left under the root mass.
9. Place the tree firmly over the soil. Adjust it for the proper height and hold it in place.
10. If I'm going to wire the tree into the pot, this is the time to do so. I start on the long side, if there is one, and loop it around the tree, using pliers to tighten the wire. I don't just twist. Instead, I first yank up on the wire and then twist.
11. Start piling soil around the outside, working it in with my fingers as I go. I fill to just below the edge of the pot.
12. Work the soil over with a chopstick (and my fingers) to fill any air voids.
13. If there is excess soil, I scrape it away. If the soil line is too far below the edge of the pot, I add some more and tamp it down. If possible, I leave the surface mostly flat, rather than sloping down from around the trunk, to allow soil to soak down rather than run off over the surface when I water.
14. Water gently but thoroughly until clear water runs out the bottom.
Finally, I place the tree back on the bench. I don't worry about sun, but I do protect my tree from wind for the next few weeks, and I do not fertilize for at least one month after repotting.