You can usually negotiate a better deal late in the month, as sales managers and sales staff get closer to their certain monthly quotas. Often the top-selling sales staff receive a monthly bonus based on how over-quota they are, and the lower-selling staff must make their minimums (to keep their job). The sales staff and sales managers will often do whatever it takes to reach their goals, to collect substantial bonuses.
Check the hours of the dealership and go in just before closing time. At the end of the day, salespeople are tired from haggling and may want to close the deal quickly. You should have pre-shopped to ensure they have the vehicle you want on the lot, so the salesman knows he can close the sale without staying late.
Never say you are paying in cash. You may not necessarily get a better deal if you tell the salesperson up-front you will be paying in cash. This deprives them of the potential of profit margin from leasing or dealer/factory financing. A cash transaction means no money on later on, so they may try to make up for it on the price.