Credit, please! It stinks to see your stuff on someone else’s journal and not see credit. Not only does it let people know where you got it from but it also makes me feel all warm and fuzzy inside, which is always nice. For icons, credit julzroguenine or graphicsbyjulz in the 'Comments:' on this page. For my non-icon graphics, I don't care how you credit me (i.e., in your profile, sidebar, or layout post, etc) just as long as you do.
Please try to comment when taking something. It's not too hard to ask, is it? It helps me know what things are popular and helps stem my curiosity about where my stuff is going.
ABSOLUTELY NO HOTLINKING! Stealing bandwidth is not cool. Right-Click (or Ctrl-Click if you are an 'evil' Mac user =P), choose 'save-as' to save them to your hard drive, and upload them to your favorite image hosting site (ex: Photobucket, LJ Scrapbook (for Paid/Perm users), or ImageShack). Important Note: This applies to all images, even those included in layouts.
One of the reasons I started this community was to compile tutorials for Opal because I'm sick of answering the same questions over and over in opal_help. So you can look for answers here, at my journal, or in that community. If you find any more tricks and hints, comment and I'll add them to my growing list of Opal Resources.