I was investigating this the other day, as it happens. Recent research from France (can't find the paper now, sorry) shows that unless you're very, very highly

I was investigating this the other day, as it happens. Recent research from France (can't find the paper now, sorry) shows that unless you're very, very highly skilled your personal bargaining power is almost zero. What was important however was how many companies were competing for you, how profitable those companies were, and the current state of the economy. This implies that a negotiation like this is just back-and-forth-ing within some window of acceptability that was probably predetermined before you walked in, and if you want to have a successful salary negotiation and get your prospective (or current) employer to redefine that window, your best bet is to a) orchestrate (some) of these external factors to work in your favour; and b) find a way to get that information into the exchanges without being a dick. They also noted that doing all this then staying with your current employer was often the best strategy because at the very least you can expect to capture some of the hiring/firing costs associated with replacing you.