First, figure out a reasonable price. No, wait: first, let go of any expectation that you'll get back close to the price you paid. That money is gone, time has passed, and people are excited about new models (for the same reasons you want to upgrade).
Second, then, find a reasonable price. I think the best way to do this is to look on reputable sites that sell used gear, like KEH, Adorama, or B&H Photo. Find equipment comparable to yours, and in comparable condition, and see what it's going for.
As a rule of thumb, assume that your gear is rated one category worse than you, as the attached owner who feels like you've been really good to it, would rate it. (File under life isn't fair, if it helps.)
Now, consider that these companies will pay you something like 40-60% of the sale value. (Adorama says they'll go up to 70%, but I wouldn't count those chickens — I'm sure supply and demand feature significantly.) For example, I see right now that KEH has an "excellent condition" Nikon D7000 camera body for $432. From that, you can figure that they'll give you about $200.
Next, there's a decision. Pick one of these:
Listen to the little voice in your head which is screaming "What? I paid $1500 for this, and it's still basically as good as it ever was! That's ridiculous!" Give up the idea of trying to make a little cash, and either keep it as a back-up body, or find a deserving relative who will really appreciate it. (Cash-starved cousin in college? Need to get some points with an in-law? What a thoughtful thing to do!)
Decide it's worth it for a discount on the upgrade you are, let's face it, going to buy anyway. (Hey, I'm not judging. Buying a new camera is fun. All of these thoughts are from personal experience, trust me.) Using one of those big retailers might not get you top dollar, but it's really painless. How much is your time worth, after all? So, go to the aforementioned KEH, Adorama, or B&H. (If you're in New York, so you could just stop in. They'll do it by mail too, possibly including shipping for free, although of course do consider that money from one place has to come from another.) You could pick another camera store too; these are just the big names.
Or, decide that you want to go it alone. Here, you pick Craigslist, eBay, or the like, and cut out the middleman. Set a price somewhere between the online listings for used gear and the fraction you'd get selling to a store — don't expect to get what they can for it, because they do add value to buyers by providing a trusted name and putting their reputation at stake. Maybe 80% of the used price from the stores; you can do an eBay search of "sold listings" (look under advanced search options) to see what people are really buying similar things for — but, don't necessarily get your hopes up.
If you decide to do #3, posting a lot of good pictures of your item will help. Here, you're basically trading time for an increased price. Consider how long it will take you to create the listing, follow its progress, and complete the transaction, and then think about other ways in which you could make a couple of hundred bucks — or just plain what else you might do with that time — including being out there shooting with that new, desired gear more quickly. It may or may not be worth it to you!
Personally, I've done all of these things; from my tone, you might conclude that I've largely settled on #1. And you'd be right. You can't sell used electronics for what they're worth in practical, pragmatic use to strangers (largely because of the whole trust thing), and I'd feel bad about giving a worse deal to a friend. So in the end, I prefer to just make sure my old digital cameras have a good home.
Or if giving it away doesn't feel quite right, there's always barter — your brother gets a nice camera, he fixes your bike, you both get good value.
