Another method to consider is using a table. Anyone who has done this with Excel will appreciate that if you are doing a quick-and-dirty block diagram, a spreadsheet will often do the trick. Color in the cells that are 'blocks' and leave the ones corresponding to backbground white. Makes annotations and resizing a breeze too. It is not for every situation but to be honest, many block diagrams are just about spatial relationships. The new table updates have opened up some real possibilities with this.