Many good tips above, and a couple more here.
Make sure your hands are relatively clean before you handle anybody's knives so that you don't grunge up a blade.
Return the knife in the manner in which it was handed to you (ie. open or closed, butt first, etc.).
If it's in a glass case, it's there for a reason. If it's out ont he open table, it's still a good idea to ask before you pick anything up. It's just common courtesy.
If you can't afford a knife, or know you're only looking, leave the knife on the table. I did, however, break this rule once, upon seeing the most beautiful damascus blade, mokume guard and endcap, mammoth fossil handle, and absolutely gorgeous piece of art. *sigh* Before I could stop myself, I stammered, "Um, Mr. Zowada, I know I can't afford that knife, but it's absolutely beautiful, and I'm wondering if I can have a closer look..." And Tim Zowada, true gentleman that he is, insisted that I do just that, and he seemed glad that someone could appreciate his wonderful work. Mr. Zowada, my brother and I all discussed features and details of the knife, and my brother ended up jumping on his waiting list, so some makers will spend the time with someone knowing that a bit of good will can go a long way towards building friendships, and developing a customer base. I guess that sometimes, it doesn't hurt to ask, just don't assume you'll be given permission.
If I know I'll be handling knives, especially nice ones, I try to carry and use a clean handkerchief or bandana or even a Tuf-Cloth to wipe any smudges off the knife just to make the effort to let the dealer or maker know that I respect their property. I'm often told, "Oh, no, that's okay, I'll do that," or "don't worry, someone else'll come along and pick it up in a minute or so", but sometimes a dealer or maker will thank me for trying to be concientious.
I'm sure there are many others out there with their own tips or secrets, but these are a few of mine.
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Don LeHue
The pen is mightier than the sword...outside of arm's reach. Modify radius accordingly for rifle.