This is something I learned long ago from a TV program called “The Router Workshop” hosted by Bob Rosendahl. I don’t think I was much older than 13 or 14, but it stuck with me ever since.

Here are two tips that help to keep your router bit from coming loose, and the first is to not bottom the bit in the collet. Instead, insert it all the way, then pull it back out by a small amount.
I’m not entirely sure why this is effective, but it allows the collet to clamp harder on the bit with that space at the end. All I know is that I’ve had bits come loose when I bottomed them out, but never after learning this.

The next tip is on how to do the tightening. I always use two wrenches and never use the spindle lock that’s on some routers. This allows me to line up the wrenches so that I can squeeze them together with one hand to loosen or tighten. This is best illustrated in the video below, and it not only results in a tighter grip on the bit, it also makes it less likely to bust up your fingers when the collet suddenly releases.