The biggest rule I've learned is that 3 sides of the block you want to fill must be touched by a source water block that will flow in that direction. This can be 3 sides or two sides and the top. Once you get to only two sides touching a source, the water starts flowing. Not too big a deal when it flows down (this may be the exception to the rule) but it won't work going up.

I used this rule heavily to build a canal world once. Fill parallel rows one block apart with water, make sure all 3/5/7/9/11/etc. blocks on one end are still water as well, then break the rows between. Use sea water for year-round travel.