I tend to be very careful when dealing with aggressive negotiators. Yeah, they might negotiate aggressively up front, then be decent down the line, but I've had so many experiences with 'aggressive negotiators' attempting to change the deal later that I generally price in a lot more scope creep up front, and I make sure to charge them for things like phone calls or emails; things I might do for free for an easier to deal with client. I'm also much more aggressive about getting paid on time when I know I'm dealing with an aggressive negotiator than I am normally.

The thing is, good negotiators put a lot of effort into putting you into a situation where it's expensive for you to walk away, then they ask for more work before they pay you. I've seen guys who are several months late paying ask for another milestone before they send the cheque. The key when dealing with these sorts is to stay out of situations where it's expensive to walk away. Give the person a credit limit, (you are, after all, loaning them money when you do work post-paid) and if they exceed that credit limit, stop work until you get paid.

(I think one needs to be careful here; I don't think it's acceptable to threaten to break things if you aren't paid... but I think it's completely reasonable to just stop, and in my experience, stopping work is ridiculously effective. Every time I've tried it I've gotten a cheque overnight.)