Sandbags shouldn't be filled more than half way full, otherwise they don't mold themselves into the sandbag wall to plug up all the little cracks and gaps.  They also have to be tied up towards the top, otherwise they don't work as good.

Not long after this Duncan took the younger Scouts out to see if we could find someone who was putting sandbags out on a pile that we could help.  The Scouts wanted to actually lay the bags, not just fill them.  Unfortunately the placing of the bags wasn't supposed to begin for 2 more days and we couldn't find anyone starting early :-(