Always a difficult one, because you're immediately prime suspect, and first port of call. Its difficult to get away with murdering the missus without making it look like self defence.

The best bet is probably the old 'seat belt' special, ejecting the seat belt just before plowing the car into a tree.

That way you can put it down to her not belting up when told to, which is after all the reason most wives get murdered anyhow.