Before you walk in their door bright-eyed; you have GOT to do some math for yourself!
What was MSRP on your car when you got it? Cut that number in half the first two years. We just did a basic appraisal (what the salesperson is going to do before handing the keys to the used car mgr). The manager will weigh current condition and miles on your car against black book (local/nat'l auction prices) to arrive at the offered value.
You also need to find out, to-the-day, what you owe on it. Subtract the appraised value from the note's balance to see if you're carrying equity or "upside-down". If the latter; it's not an advantageous move for you. However, any equity can help boost money down in addition to and dealer incentives or mfg's "cash back".
If you can trade into a vehicle with better features for the same payment; you'll only be extending the number of years you're responsible to pay on it... but in a 'better' car. Those flyers are distributed solely to "drive foot traffic" in the door and create more oppotunities for their sales force to close deals. The same way a shop might offer you a free oil change in order to get permission to walk under your car and look for problems to fix that you may not have been aware of.
If you're tired of you current car or worried about failing equipment on your car, then neither sales/service trying to sell or fix you up is a bad thing... just educate yourself a bit before walking in!
"I'm gonna sing the doom song..."