There may be some general rules of thumb to follow,
There is one big one: Have the willingness to walk away. That's the one and only weapon they have zero answer for. Don't use this tactic as a bluff. It won't work. You have to really mean it, and that takes some guts.
Just remember, they see people like you every hour of every day. Every tactic you'll try, everythng you say, they've heard it all a dozen times over. You will not fool them and you will not say anything new. Walking out is the one thing they can't control and cannot fight, except by dropping their price.
If you decide to walk away, be firm and don't look back. Don't hesitate. Don't vacillate. You might just find a call from the salesman on your answering machine when you get home, or you will be physically pushed back into your seat, meaning you're going to get your price. If they let you walk, your price was unrealistically low.
Having said all that, you've also got to be realistic in your expectations. If a car is a hot seller, the ball's in their court and you will be unlikely to get any sort of break on the price they want for it. If you get up and walk out because you can't get your price on a hot seller, they'll let you go.
If at all possible, avoid financing and trade-ins unless you have no choice. Those two things are where they get most of their money. Do your homework, then walk in with a set, ON-THE-ROAD price, and stick to it, asking the salesman "what can you give me for that price?". On-the-road means the final bill after ALL charges of ANY kind are added in. Don't budge, and don't let them tack on other charges to your on-the-road price at invoice time. If they do, get up and walk out.
The above approach gives the salesman the impression you're serious, that you have the cash, and he will be hard-pressed to let you leave without finding something that will fit the check amount you'll write.
He's more likely to hunt around for you for something to fit your price. You may end up with a less-than-popular options package, or an unpopular color, but at least you've got your price.
Oh, and one more thing: Forget this "below invoice" nonsense pushed by some consumer groups. It's a mug's game and you won't win it.