How do I fix a wobbly mailbox post?
You have a couple of options that I can think of, assuming that the ground around the post is fairly firm:


Wedge something in next to the post.  (thin rocks, cut down cedar shingles, etc.)
Fill in the gap next to the post by pouring in sand


If the ground's soft, you'll need to re-compact the soil around the post.  (and if it's not soft, you can always loosen it up, then re-compact it, as in some cases that may be the easier fix)