Amen BigLou, Contact cleaner works better than anything else. I do the exact same thing you do, I spray it onto a paper towel or shop rag first and then wipe the plastic. I thought this was my little secret, but it appears the word is out. Contact cleaner does an absolutely amazing job with little to no effort.
I ride in coal dirt, so my plastic turns black and I mean BLACK !! Especially when you wipe out and the black is dug into the scrapes in the plastic. No worries, because the contact cleaner will restore it to bright white.
Of course my other little secret is to replace white or light colored plastic, with something darker. I usually replace the plastic and seat cover with something black if I cant find it. Black plastic always looks clean !!!