I'm no expert but I'd say put a few coats of putty primer on before the base coat/colour. That gets rid of the dimply effect some plastic trim has and also gives the paint a better surface to stick to. Give a day between each coat then lightly sand with 640 or 800 grit wet & dry sandpaper (you'll get this from Halfords). Then a few coats of your base colour and then a few of your finishing colour. Better more thin coats than fewer thick coats, hence the cracking, I think. Each coat should be given at least a day to dry, preferebly not too cold an environment either. Then put on a few coats of laquer and sand using fine (1200 & then 1500 grit) wet & dry sandpaper. Always keep the sandpaper wet, it gives a better finish.
I've painted a few computer cases using this method except there was a helluva lot more coats/sanding as I wanted an almost mirror finish.
If anyone knows any better feel free to inform, like I said I'm going to do mine soon too.