There are three basic thing you really must do when selling your valuable jewelry in order to get the best price. Without them, you are pretty much throwing yourself to the wolves.
1. Know what you have. Do you really know the value of what you have? Never mind what Uncle Joe said it was worth, and forget about the hyper-inflated-superduper-buyback guarantee from the diamond store. (They'll pay a small fraction of the original sales price)
Do you really know what similar items are selling for in the type of market you are putting it in?
2. Realize your market. The last few words typed above are critical: "the type of market you are putting it in". Basically what this means, is how fast are you trying to sell it? If you want instant cash, you'll have to sell to a dealer, this is going to bring the lowest price you can get. If you are putting it on eBay for a fixed price it may take longer to sell, but you can likely get closer to the retail value than if you were to go with a dealer. And somewhere in between would be an eBay auction or a live auction, where you let the chips fall where they may. It's a gamble, but it could pay off and will likely bring more than a dealer will offer, but less than retail.
3. Have a bottom line. No matter what you do, you are going to be unhappy if you don't get the minimum you're looking for. Know what you have, then set a bottom line based on that. Of course this requires that you be realistic and price your bling competitively in the marketplace. Nothing can help you sell an overpriced item!