If it is a soft plastic like vinyl on your door trims then I advise you use a plastic primer so the paint adhere's to it better also so it dosn't mely it. If it is a hard plastic like dash bords then clean it with (also clean 'soft' plastic) wax and grease remover. Thinners might melt the plastic so I wouldn't use that.
When you paint, do it on a non humid day that has plenty of sun, once painted place the painted item in the sun for a while, this will help make the paint finish more shiny so it will blend in with the rest of the car.
Whether you take my advice is up to you, this is what we told people when I worked in the paint/panelbeting supply store (not scotties/supercheap/repco etc).