This evening, I had the unfortunate luck of someone completely taking out my wooden mailbox with their side-view mirror, leaving only the post. It was a total accident, and he was quite apologetic, and I had my mailbox back up in about an hour with the help of some 1 1/4" brads.
The mailbox is now firmly reassembled - the only trouble is that he knocked it hard enough that the post is now wobbly! I'm not sure how deeply it's sunk in the ground, and from the way it wobbles, I'm pretty sure it's just a ~4"x4" wooden post sunk into the dirt. What is the best way (where best in this case means quickest and cheapest, without being slipshod) to firmly fix that post in place?
The post may be broke off in the ground. If it's really wobbly, it won't hurt to try and yank it up and take a look. You could try to sink something like a 1x4 down long side the existing post. Of if the existing post isn't too damaged, maybe dig around it and pour some post hole cement in. It's like $5 for a 50lb bag at most big box home stores, you just dump it in the ground and add water, no special mixing or anything.
If you end up replacing it, use something that is pressure treated so it won't rot quickly, although anything in contact with the ground will rot eventually.
I would pull the post completely. If it is showing some rot below ground (possibly why it is wobbly) I'd replace the post with a new one or cut off the rotten part. Then buy a stake meant for this purpose. I used one years ago that was just a heavy steel stake, driven into the ground with a sledge. The wooden post itself is then screwed to the stake, so all wood stays completely above ground. A quick check online shows an anchor that you screw into the ground, or this stake that is driven by a sledge.
I found a product called Securepost on Amazon that is made to straighten mailbox posts. It
is a plate that you dig down 4 inches and wraps the post and then pound 6 stakes through it. It worked for me.
Dig a hole larger than the post and put gravel into the hole until the post sits on top of the gravel at the right height. Then add gravel around the post a little at a time and keep packing it in. The gravel is harder to displace than dirt, and it allows the water to drain away from around the post so it doesn't rot as quickly.
Dig out the post set a new one in its place. If there is concrete I use a demolition breaker to break it up, then remove concrete and add a new steel post followed by concrete. Dome the concrete to shed water away from the post.