Here are two madonna's on ebay right now- the first one is an authentic "Love Madonna." Not as perfect as I've seen it, but pretty close.
http://www.ebay.com/itm/Madonna-AUTH...item2329619bcd
Now click on the one below and see where she started to sign "Love" and then just rushed the rest. I would say this is probably authentic, but gosh, it's just a scibble that anyone could do. So for me, I want the nicer example because not only does it look better, it's certainly tougher to forge.
http://www.ebay.com/itm/Madonna-TOP-...item2329619f92
It's pretty sad that I've seen enough of her autographs to say that the second one is real, but the more I look at it, the more I say it's genuine. The slant is right, the way she starts the L and the tail at the end of the a....