Can't emphasize having good slates enough. Keep in mind that later on you will have to read them from a frame a few millimeters wide or on a 13 inch screen at low resolution. Of course, there is no way to tell this is the field but if there is any question, remark the slate. Chalk slates tend to be hard to view at pixelized low resolutions on a computer so I don't recommend them.
Make sure every slate is full in the frame and legible. Do have the slate fill the frame. Do clap crisp and sharp but not hard. Do clap onscreen, please. Do mark every shot, no matter how small the cutaway, no matter how rushed you are, whether it has sound or not. Do tail slate upside down. Don't move the slate offscreen until you've clapped. Don't clap MOS shots.
When you mark the slate verbally, speak clearly and evenly. There shouldn't be any need to shout.
Nonelectronics slates can be used but it does make the sync job a bit more difficult and time consuming. The technician at the lab has to use a special "audio layback" workstation to match each clapstick on the videotape to the sound of the original audiotape. The technician then aligns the first frame of the clapstick coming together with the sharp hit of the slate. Audio is then dubbed for the length of the shot. At each take this process is repeated. However, if you keep good records, the time (and therefore, money) involved in this task it greatly reduced.
If you are using electronic slates, then syncing sound at the lab will be quick, simple and reasonably priced.
An electronic slate has a red LED display with SMPTE/EBU timecode on it large enough to be filmed. The advantage to using a electronic slate is that a 16mm camera can film the Time Code display at the beginning or end of a take and the resulting image can be eyematched to the audio tape's Time Code readout during syncing. There are two different methods for generating this time code, smart slates and dumb slates.
Smart Slates generates its own time code display which is jam synced (matched) to that of the audio recorder.
Dumb Slates display time code from the audio tape recorder by a wire.