First, if you can't loosen the seatpost bolt, then cut it and remove it. The easiest way is to get a disc cutting tool that mounts on a hand drill. Run the disc in the slot of the seatpost collar to cut the bolt.
WD-40 is OK, but I find that Liquid Wrench works better for freeing up frozen parts.
Next, check to see if the seatpost and the post-tube are the same metal. If the post is aluminum and the frame is steel or visa-versa and you can't get it to move, give up.
Keep the saddle on the seatpost. If you can at least get the seatpost to move by turning the nose of the saddle, you have a chance.
Sit on the floor and put the bike on your lap. Have the saddle in your hands. Put your feet on the inside of the frame - one foot on the down-tube and one on the chain-stay. Use your whole body strength to pull on the saddle while you twist it back and forth.
If you have children, don't let them be around you when you do this... You don't want them to repeat what they hear from you.