You can prevent Flash from automatically loading in Safari using Safari Stand. Install it, launch Safari, go to the Stand menu, choose SafariStand Setting, then click on Advanced. Check the Load Plug-in Manually box in the Flash section and restart Safari.
Now when your browser encounters Flash, you can click the box where the Flash would have been to load that bit of Flash. You can also add exceptions to the site alteration area -- for example, you might want to let Flash automatically load when you visit youtube.com. (This tip originally appeared on my blog.)
[robg adds: Many of the browsers offer some sort of Flash controlling plug-in. With Firefox, I use FlashBlock to control how Flash behaves.]

