Common yet still tough situation. In my experience, all you can do is thinly draw the line between being polite and pushy. Just the usual - email and/or call - ask about when can you receive the payment for your service. Lay out the time frame of when you performed your job; when did they say they would pay you, etc. Maybe you can also say something that you look forward to the payment because you need the money for a future project in (give a specific month or time frame).
But you do need a lot of patience in talking to them because even if they owe you something; you still have to act like you're asking them for a favor (especially for big companies).
And oh, if that client caused you that much trouble, take note of the name and think twice before doing business with them again.