The simplest way is to just make a hole in the block and pour water in it every so often. Try to see the thread about YTong nest building..
I saw some dude had carved a hole in the top-side of his YTong-block and put a sponge into it, so the sponge would hold water and slowly distribute it.. It looked neat, and you wouldn't spill, but I've never tried it..
Anyone think it's a good solution?
You can use this: blumat for house plants.
http://blumat.pl/shop_content.php/language/en/coID/13/product/Dla-ro--347-lin-domowych
http://blumat.home.pl/pics/blumat_Room_M2.jpg