Leave it till the last week of the month also as they have targets to hit and this is when they discount the most.
Sarnie is spot on with the buying signals bit.
Also be enthusiastic about Paint protection, accessories GAP etc, they will discount further thinking they can make the money back on these things. When you come to the final figures leave it for a day to think about it (preferably three days before the last day of the month, in case they are very close to target) and then do the deal for the discount price minus the extra's.
My old colleagues in the trade tell me that they are 40% down on last year and struggling heavily so now is the time.
All dealers try for a 2k mark up on used stock over 5k in value, and most new cars have at least a bag in them, also if it is a pre reg, they have upto 3k in them. so if you can check how much it was originally up for if its used or if its a pre reg if its new.
They will also be looking at getting 1-2k profit for your trade in if you have and its worth over 5k so bear that in mind when negotiating.
Just me 2p's worth and good luck
Jay