Negotiating Teaching Assistant salary,
In general, TA salaries are not negotiable, but are instead set by a department or school for all students. There may be some "wiggle room" in special cases: for example, if you have already served as a TA and have experience, there may be a higher level of pay available under such circumstances.

In the event that your school allows for negotiations, I would say in any case that you would have to provide a solid argument for why you deserve more than the standard salary. Do you already have teaching experience? How much? Where?