It crossed my mind this morning that we are in August. September is the other month of the equinox (exactly half a year away from Shakespeare's "Beware the Ides of March"). March and September are the months where the Earth tilts just more than halfway to or from the Sun. So September and March are the months where gales are most likely and trees are most at risk of being blown over.
I hope this does not happen to you, but if it does, here is how you can save a leaning tree (...and you might want to take preventative action on trees that are leaning now). By the way, if the tree falls over, cut it up for firewood - it cannot be saved. These instructions only apply to trees that are LEANING
1. You need to stabilize the tree (stop it from leaning more). The best way to do this is by propping it up. Anything long enough and strong enough will do. Your local friendly engineering workshop will make you up a Y shaped prop, or you can use a stout bit of timber (cut down floor or roof joist from a skip, 3"x3" fence post, something like that). An Acrow prop would be the best if you can get one - incredibly strong and capable of being made longer or shorter while in position.
Ideally you want to give the prop a foundation, so it does not drive into the soil under the weight of the tree. Obviously a concrete footing is best, but a paving slab will do the job, even a large rock. The prop needs to be as close to right angles to the trunk of the tree as possible and held really tightly by the foundation. Concrete does this all by itself. For anything else jam the prop in by hitting its base towards the bottom of the tree so it slides across and then gets stuck on the foundation.
2. Ideally in winter (but if the tree is leaning badly do it now) completely cut out one of the large branches that is causing the tree to lean. This reduces the weight that is causing the tree to lean. You can do another the year after and so on. This winter or next spring, prune the side AWAY from where the tree is leaning and prune it [B]hard[/B].
Sounds mad, but it will cause new growth on the "good" side which will help stop it falling over.
3. As a minimum, the tree should stabilise and more root will form away from the direction it is leaning which will anchor it. If it is relatively small, you can gradually force it upright as the tree reshapes. Keep on pushing it up and in a few years it will be vertical.
But let's hope the winds don't blow.
Watch your plants grow, and Enjoy!