A rule of thumb that I read is to add 20% to their intial offer, that is about the max of what they are willing to pay you. If that number is much lower than what you want then you probably should look elsewhere. Also, don't be afraid to ask for non-salary benefits like extra vacation days, life insurance (if you have kids), no-travel clause, etc as the company negotiator often has more leeway on these things and a hard "no higher than x" limit on the salary.