3) get a canon st-e2 infrared transmitter. more expensive, but easy since it will work the ettl for you and is pretty much plug and play with your 580 ex, however limited by the fact that you need to have a direct line of sight to fire though.
4) get some pocketwizards. these babies are tried and tested and work beautifully. radio transmittion means you can have them behind objects and they work with ettle so no figuring out flash ratios before hand. i've had a chance to borrow my friends ones once and they synced perfectly every single time. downside... ridicolously expensive.
A few ways to it,
1) use a TTL cord (limited working distance)
2) use your on board flash to trigger another flash with built in flash function (here you are firing 2 flash, if you want just the slave to fire, you got to use a device to cover the on board flash so that the it won't light up your subject, only to trigger the slave unit.
3) Use a wireless transmitter & flash trigger to activate the off camera flash. Here you can have as many flash as you want provided the flash are within range of your transmitter(either with wireless trigger or with slave option)