While this tip isn't on par with some of the fantastic tips on this great site, I nevertheless thought I'd offer it up, as I've not seen it here. [Editor's aside: All tips are equals here at macosxhints.com! After all, there's a wide range of experience levels out there, so every hint probably has some value to someone in the audience.]
When pasting a custom icon in the OS X Finder, make sure that the given volume (or folder or file) does not already have a custom icon. If it does, you will get a blank icon, requiring you to either log out and in or re-launch the Finder.
The way around this is to first cut the custom icon, thus reverting the icon to its original default state. Now you can paste away!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020228014410576