Even in these days of 'automatic everything' there is still much to be said for that golden industry-standard piece of equipment, the CLAPPER BOARD. It make sense to visually and audibly mark the exact synchronisation points in your film rather than relying totally on fallible electronic equipment (What, you don't think electronic equipment is fallible?  In that case you've never argued with a computer). This inexpensive item is surely a 'must' for any director/cameraman worth their salt. 

If you're shooting a lot of takes it always helps to have a slate before each one when you come to the edit. These clapper boards are for use with chinagraph pencils.