With a mixture of soda and water. (toothbrush is what I use)
(if anyone has a better mixture, you're welcome to comment)
That will neutralize the acid effects to a certain extent.
But if you don't replace the battery, it will do no good.
Remember, this is my opinion...
I'm not someone who works on hardware for a living, and probably a good thing too. 
First get he board out, then this is exactly what you do. Ditch the battrey, clearn the board, give it 48 hours dry time after you pat the excess water off. Then replace the battrey with a new one.
Lucky you that everything still works. But GET THAT ACID OFF THE BOARD!
I saw a hint of that yellow stuff on my A4000 battrey and ripped the puppy right off. Yes, right now I am setting the clock by hand every time I turn the machine on, but that is cheaper than a new motherboard, as if I could FIND one.
