I'm a keen user of FW1 spray cleaner. It removes road grime effectively and wipes off to leave a shiny wax. Be sure to read and follow the instructions (for once). It's good on windscreens as well as the plastic. It's not cheap at $20 a can, but I get about 20 cleans of the bike out of it, so I reckon it's good value. They sell packs which include an upholstery cleaner and tyre spray, both of which are wasted on a bike (and who cares about spraying their car tyres?)
So what I do is haggle a deal on a few cans of the cleaner. They are franchisees, so can do a deal if they want to. They operate in shopping centre carparks. Check weekly locations here:
http://www.fw1.com.au/cgi/prosales.fcgiAnd no, I don't sell it, nor do I personally know anyone who does.