Drop the phrase "just kidding" from your vocabulary. It subliminally hints at insecurity ("If this joke's not funny enough or they take it the wrong way, I'll provide the easy way out.") It gives away the mysteriousness of a joke - you want women to wonder "Is he serious?" and determine the answer themselves. If they don't get the joke and find it amusing, you either need to improve your sense of humor/joke delivery, or they need to buzz off. Either way, "just kidding" should never need to be used!