motown themed party ideas?!
Music would seem to be the obvious focus of your entertainment. You could do karaoke, or if your guests are dressing up, have them come prepared to "perform" a popular song from their artist. You could have someone record this and later give out DVDs to everyone as a belated favor. Dancing would also seem fitting.
You could do a costume contest, trivia games, etc. You could have a black sheet decorated and hung as a backdrop for pictures and have a parent take a pic of each person/couple/group with a polaroid or with a digital camera and give them out later as favors.
As for decor, I'm sure you could find this theme online for party products or you could simply use old records hung from the ceiling, big music notes cut out from cardboard and painted black, and lots of sparkle, like gold lame', sequins, etc. Get some gaudy fabric and drape it around, use for tablecloths, etc.