I use the all-left-hand signals--pointing left when I'm turning left, and pointing up when I'm turning right (I'm not sure what the equivalent may be in countries where you drive/ride on the left-hand side of the road).

FWIW, in the UK (where we drive on the left), pointing up with the right hand when turning left would not be recognised at all. Bike signals here are definitely right-arm-out to turn right, left-arm-out to turn left. (as in the Highway Code (PDF link). Note that that does show a right-hand signal for drivers -- circling the hand to signal a left turn -- on the grounds that you can't see a driver's left hand, but the motorbike/horse/pedal bike left signal is left-arm-out.)