Mark U , Oct 29, 2006; 05:29 p.m.
If you don't want to add another flash, then you can always get Canon's ST-E2 wireless slave controller:
You have to take care that the slave photodiode on the 430EX can see the IR signals from the ST-E2.
You could also buy another flash (e.g. Canon 550EX, 580EX or Sigma 500 DG Super) that is capable of acting as a wireless master. Although it would need to be mounted on the hotshoe or OCSC2, you can set it so that it only controls your slave and produces no flash itself (other than signalling flash, which doesn't affect exposure) - and you'd have the benefit of a second flash if you later wanted a more complex lighting scheme.