I use a bit different tactic.
First, I sell my vehicle outright when I can. I WILL NOT sell it until its paid for. If you can't afford the one you have, how will you afford that and the new one? Thats what you are doing. Wrapping up the old in-equity with the new in-equity. Hang on till its paid off. Then you are closer to "getting on top". The only time I trade in , is if the car is under 60,000 miles, so it doesn't get wholesaled. You can save an amount equal to the taxrate, if you can get the same trade value as open market. High end cars typically don't sell well in private sector.
Second,
It sounds cold, but if you can't pay it off in 36-48 months, it costs too much. This length of time will vary depending on warranty. And the milage /year you drive. Whats the sense of having a "new car payment" with repair bills? Nope. Not for me.
Third.
Dealers: Research the car, going rate prices, etc. KNOW the facts. Go into the dealership ALONE, look for the Newbie salesman. The sales mgr will be trying to help him make quota. First sit down goes like this: "I will pay $xx,xxx. for that car. If the sales mgr balks, then tell him to save EVERYONES time, and come see me. I WILL NOT wait for more than 5 minutes. I will leave. (and look at your watch) I don't have time to jerk around, I will buy a car today. Here, or elsewhere." At this time , he's all flush, and runs to the S/M. 5 minutes goes by, and he comes back alone with a counter offer. You tell him " You didn't hear me correctly, I said if he want to haggle, get over here. " Then stand up. The salesman will have a kitten. The S/M will see you, and come over. YOU ARE DRIVING NOW ! You control the coversation from this point. He will mention his cost of the vehicle. You ask, if that includes incentives, hold-backs (a Biggie!), or other programs available. He will then know what he's dealing with. It helps to research incentive programs available BEFORE you go, to see if your getting stroked. You can also mention a program that he "forgets". These guys HAVE to move vehicles, so they need to negotiate EVERY deal. The longer they sit, the longer thay pay "floorplan" on them. You can also mention that about a vehicle you have seen sitting there for a month or so. Say"Gee, that Red Silverado has been sitting there for 2 months (if it had) The floor plan must be killing you!"
I've bought 12 new cars, and the WORST I did was 12% off the sticker. Best was 26%. My lastest venture was upgrading my 02 Tahoe for an 04 that was loaded up,even a DVD player, for $7,000.00 (Thanks in part to a GM rebate of $5,000) The old one had 61,000 miles. I'll take that.
For right now though, I'd get into positive equity before moving.