Bikes really depreciate fast. I think Mike is about right. If you sell, you get about half what you paid (that’s for a gently used 2-3 year old bike – any older than that and it will really plummet), but the bike shop has to tune it up and stand behind it, so they really can’t give you too much [even if they love you in a non romantic way]. I bought a slightly used frame for $1600 that was normally $2600 and it looked virtually new and it wasn’t very old. The dude was trading it in on a $10,000 dollar bike, so maybe he isn’t so sensitive to the loss
Things you can do to minimize your loss. You could get some cheap wheels to throw on and keep the nice ones, save the bike for rainy-day riding, or even cannibalize it for parts. You could sell it for the same price that the bike shop will give you to a friend or relative to start them riding (at least it will make you feel good). Or, like was suggested, sell it on Crag’s list or one of the bike forums.
The only way to really save money is to buy a bike and then ride the wheels off of it. I ride a pretty fair amount and I usually get maybe 7 years or so out of a bike. You can also, much like a car, buy a 1 or 2 year old bike and let the first guy take most of the depreciation. I swear some people belong to the bike-of-the-month club (their loss, your gain).
If that’s not your cup of tea, a great time to look for a bike is November or December (huh? just like a car!) when the new models come out. The LBS often gets special deals and incentives from the manufacturer that they can, and usually do, pass on to you. I bought a MTB the first of this year that was a 2009 model on close out and I saved 30% right off the top. I would have never spent what the usual retail price range since it was kind of expensive, but with the discount, it wasn’t so bad. Usually there isn’t that much that changes from year to year and so for me it’s worth getting a closeout and this is actually how I usually buy my bikes.