Check edmunds.com to see about any rebates that aren't published to the public. Factory to dealer, etc. That can easily get you under invoice. Edmunds also has good pricing threads for the vehicle you're looking at. Great to help you figure out what the market price is for your area.
It may or may not be worth haggling over the holdback since it is there for a legit reason.
After that, e-mail/internet. Bought my last two cars via e-mail/internet. Easy as pie. E-mailed or contacted via their website every dealer I was willing to go to. I only responded to those offering quotes. I ignored all those asking me to come in with no quote. I also ignored those whose quote was way too high. After a few hours, it was clear there were about 3 dealers worth negotiating with. Just e-mailed back and forth a little. Eventually, it came down to one because he would negotiate accessories I wanted as well. He ended up negotiating with himself.
All in all, it took roughly 24 hours to get to the final dealer. That was with both cars. For this last one, I spent a few more days cuz I kept wanting more accessories and would negotiate with him on those. If I knew exactly what I wanted, like on my first car, the process was done within 24 hours.