WD-40 won't work, there are penetrating oils made for just this thing. I've found the key is to get the penetrant inside the seatube- pull the BB or put it in through a water bottle boss, keep the frame inverted for a night, then take off the seat, lightly clamp the post in a vise and turn the frame..it will slide right out.
You know what works best to break the bond between the post and the seatube? Coca-cola!