1) Get pre-approved through your financial institution. It does no harm to let the dealer know since it conveys that you are indeed a buyer and gives you leverage in deal. From experience, plenty of people who state "I have good credit" in fact do not, so the dealer always expects the worse. AFTER you have negotiated price, then you can ask the dealer to see if they can get you a better rate (You don't have to disclose your actual approved rate if you catch my drift...approved at 3.5% then tell dealer you got it at 2%) and then go with the better rate.
2) Use the internet to find the EXACT new vehicle you want with options to determine invoice price. Normally the automotive manufacturer sites have a copy of the window sticker you can download once you find the vehicle. Basically just build out the vehicle you want and then find the exact match(or closest match) at a nearby dealer. Go to truecar.com and spec out the exact same vehicle(same options, etc. and match up MSRP) to obtain the invoice price (I confirmed the truecar.com invoice price on my 2012 Rubicon by using the jeep forums as well). Also use the internet/manufacturer site to determine if there would be any incentives to apply. If applicable, deduct those from the invoice price. Determine what you want to pay (invoice plus + $200 for example) plus taxes, title and fee.
3) Now that you are an EDUCATED BUYER(not a tire kicker or shopper), go to the dealer that has the vehicle you want on the lot and PURCHASE THE VEHICLE.
A dealer will not let a buyer leave the lot if they can do the deal. We have purchased 2 new vehicles in the last 3 years in this manner and it really is that simple.
All this waiting until the end of a month (or quarter), getting lowest prices and then shopping other dealers (who do not have the vehicle you want on the lot), and playing whatever games is basically a waste of your time and the dealers.
Just my .02 and good luck.