Your trade-in is worth what someone will pay for it — but "someone" doesn't have to be the dealer sitting across the desk from you. Knowing your car's real value, and treating the trade-in as a separate deal, keeps you from losing money in the shuffle.
Look up your car's trade-in value using a couple of independent valuation tools before visiting any dealership — searching your make, model, year, mileage, and condition. Getting a range from more than one source gives you a realistic number to compare against whatever the dealer offers.
Some used car buyers and online platforms will give you a real, no-obligation cash offer for your car, valid for a set number of days. Even if you don't sell to them, having a real offer in hand is the strongest negotiating tool you can bring to a dealership — it's a number they have to beat, not a guess.
As covered in negotiating price, agree on the vehicle's out-the-door price first, as if you're paying cash with no trade-in. Only after that's settled should you bring up your trade-in. This stops the dealer from quietly inflating the new car's price to "give" you a better trade-in number that isn't actually better.
Being upfront about dents, mechanical issues, or high mileage doesn't hurt your trade-in value as much as being caught hiding something during inspection — that tends to trigger a bigger cut than the issue itself would have. A clean, recently detailed car with maintenance records also tends to appraise better than the same car without them.
Private sales typically bring more money than a trade-in, since the dealer isn't buying low to resell for a profit. The tradeoff is time and effort — listing, showing the car, handling paperwork yourself. For an older or high-mileage car, that hassle may not be worth the price difference; for a newer, in-demand car, it often is.