There are many variables and it is not possible to draw up rules that will ensure success.
It is important that your body language and persona conveys enthusiasm for and knowledge of Hemes.
SAs have a feel for customers who are resellers.
Be up front from the start and let it be known that you want to buy a bag for your wife.
Where you live is a factor and in some parts of the world you will be clearly told that If you spend X you will be offered a bag. At leat you know where you stand. In other places people spend X in the hope of being offered a bag and wait for ever.
A Birkin or Kelly is not an investment, so do not let that idea influence your choice.
Admittedly some will hold their value and the price new continues to increase, so it is better to buy now rather than later.
You would be unlikely to recoup your expenditure (especially if you factor in dealing costs) if you try to sell in the short term.
Some stores will tell you up front that you need to establish a purchase history. Some will take your wishes and either call or never call. Once in a while someone will say "Yes, we have one. Here you are." In general, though, it is easier to get a Birkin if you love Hermès and don't just want a Birkin 
