How to plant trees together so that they don't look contrived.
There is nothing straight in nature.
This is the most important notion when planting a natural garden. Birds spread seeds around, baboons spread seeds around, water spreads seeds around. These seeds arrive where they are deposited and sprout where the conditions are favourable.
Have you ever noticed how many trees grow under or near fences? This is because birds perched on the fences deposit the seeds in their feaces.
I always try and emulate nature when planting. Go have a look in the veld and actually measure the spacing of trees you will see that a lot of them are planted right next to each other some a few feet apart this creates a nice random layout that is not regimented in any way.
Mix trees of differing heights to create a tapestry of texture and form. Some trees are designed to fit under the canopy of other trees. Wild Olive - Olea africana for instance fits under the shoulder of White Stinkwood - Celtis africana.
If some of your trees fall over a bit, don't straighten them they will find their own way in the world and look a little different in your garden.
Don't prune. This is a very difficult thing for the average gardener as we like to think we are in control of the garden. I think that a garden should be allowed to happen. There is an ebb and flow and if you interfere too much you will kill that natural look you might be after.
Don't rake up leaves. If you leave them where they fall you create a natural mulch that traps the moisture in and feeds the trees with a natural compost. This attracts more creatures to give you that balance that you are looking for.