The safest way is to use clear packing tape as has been mentioned.
Pull off a strip of clear tape about 2 inches longer than your luggage tag. Lay the tape sticky side up on the table, counter, etc. then lay the luggage tag on it, pull off another strip of tape, and cover the upper surface of the tag. Bingo--the tag is enclosed in clear tape. When you attach the tag to your luggage, staple the ends together but be kind enough to cover the staples with a strip of tape so the people handling the luggage wont stick their fingers. I always put 2 tags per bag just to be on the safe side.