I start by assessing my object for sale, then jot down a few descriptive words that pop into my head. I also use both my Thesaurus and Dictionary. The EtsyRank site has a color thesaurus also. Tags can also include what the item is used for, its history, and anything that might relate to it.
Example: A rhinestone necklace can include tags like Vintage, Retro, 1950s; Color, Aurora borealis, Ruby Red, Emerald Green; Prom jewelry, wedding jewelry; Designer, Country of origin. Superlatives like Dazzling, Sparkling and Gorgeous can be used, too. Pretty limitless once you get started!