1. Call / email for price quotes - The biggest dealers (due to much higher volume/scale) tend to have no problem quoting you a price over the phone. Those that can't compete on price, tend to tell you to come to the dealer for a quote. If a dealer doesn't give you a price over the phone, ask them if they would like a call back once you have a price quote in hand so they have an opportunity to counter.
2. Be flexible - Don't preemptively narrow down the inventory available to you with specific features/colors. Get price quotes for the make/model you're interested in first and then start asking about feature/color/service differences, etc.
3. Set a tight timeline - I'll usually call around in the morning and tell dealers up front I'm going to buy this afternoon. I tend to get the most competitive quotes this way and it gives the dealer an opportunity to quote a price based on the inventory on hand rather than a general quote.
4. Get counter offers - Once you have a set of price quotes, call the dealers again and get counter offers for your best quote. Some big dealers are so confident in their offer they'll encourage you to call around and see if anyone can match it. You WANT dealers to start dropping out and saying they can't meet that price. If everyone counters your lowest quote, that price quote was too high.
I've learned that dealers usually know which competitor in their area offers the best price...you just have to spend a little time wringing that out.