Tags! Tags are one of the smallest, but most important ways to get someone to find your items. There are so many tags that can be used to describe your unique item(s) on Etsy (as well as flickr.) Usually when tagging an item I first browse the suggested tags listed under the chosen category for my item. I post a lot in the Geekery category. My top 3 tags I select from the suggestions are: Robots (duh lol), Toys, and weird. After I have selected from the suggested tags I add my own. For example lets say I was adding tags to the item listed below, my "Happy Birthday Robot.
Tags I may include for this item would be Birthday, Gift, Silver, Cute, Fun, and Unique. I always recommend adding the main color of your item as a tag. You'd be amazed how many people just search by color. If your item is on of a kind (OOAK) add that as a tag! You should also put your shop name as a tag as well, It makes it easier for people to find your items via search engines like Google shopping.
Everyone has their own unique way to tag. The best thing you can do is look around at other established sellers and see what type of tags they are using on their items.