Rapport. Mutual respect. Friendly demeanour.
All important aspects to getting a good deal. Be nice. The cynical will call it using human nature to your advantage, but the fact is that you are far more likely to get a good deal if the sales person actually 'likes' you. As such I wouldn't recommend the walk in walk out routine, since it's disrespectful and requires the sales person to swallow a lump of pride to deal with you. It does immediately put them on the back foot, but remember they are still much better at playing 'the game' than you and no-one likes an arrogant or disrespectful punter. But it it works for you, fair enough, it's certainly one way of getting in a new car!
I used carwow etc. to get basic prices. Those dealers whose prices I liked I started emailing about potential deals, pricing, specs and pcp terms. This gets the conversation going. Don't waste their time though. Mark yourself as a serious buyer and keep your initial questions relevant. Stunted, disinterested and unhelpful replies will quickly weed out the dealers you can't work with.
Conversation leads to rapport, and then you are both invested in the relationship. Human nature and behaviour is a topic every sales person studies in order to sell more cars, but it's important to remember they are human too. If they like you and have invested a bit of time and effort into a relationship with you, then they are more likely to want to offer you a good deal.
You already know what sort of discounts are possible from reading this site. Aim for them. Try to exceed them if you can, but don't get caught up in them. The aim is to get some money off, and deal at a price that YOU are happy with.
If you are happy, then you've won the haggling game, but try to review the contract you settle on dispassionately. Human nature is at play again, and emotions are easy to manipulate;
You can walk away from two different dealers with exactly the same deal. But depending on the relationship, you can walk away really happy, or you can walk away feeling like you just had your pants pulled down by a smooth talking charlatan. That's why you need to review the numbers and the detail in your own time to make sure you are happy with the deal you have.