Dealers tell you any old nonsense to maximise their profit, of course. So if you have sat nav "ooh well, makes no real difference sir", and if you don't then it's "ooh, well, you if you only had satnav I could be a bit more generous, but then of course <insert car colour here> isn't that popular anyway you see".
I guess we'd all do it if the position were reversed though.
The only really successful strategy I've ever found is to say "in that case forget the whole thing" and actually mean it. Last time I did that I did really convince myself I no longer wanted the deal. The dealer subsequently rang me three times at home, getting shirtier each time because I kept saying "no longer interested mate". Finally he rang and said "ok, you can have the original deal you asked for" and was as nice as pie from then on. But you have to really mean it when you say no. After all, there's always another deal round the corner somewhere if that doesn't work.