Cheapest half-decent way to get fancier light for portraits is to hold a reflector uo to one side of the camera, and twist the flash head so it points at the flash head..
BAK..
I think straight-on, direct flash is considered horrible so you'll want to get some sort of diffuser gizmo and/or bounce the flash. reflectors can help shape the light (even foam core can work sometimes). you could try combining flash with available light (say if there's a window nearby)..
This video talks about on-camera flash:http://www.shootsmarter.com/video/ocf.html.
You could get a slave flash and then have that somewhere else and then your 580 can trigger it (and also serve as onboard flash)..
For a studio, though, I think I'd prefer to just have some regular lights with softboxes (or some people like beauty dishes or umbrellas). we normally use three (one for key, one for fill and one for hair/rim lighting). we use hot lights which are always on so you can see what you're doing but they're a pain on a really hot day! if you look at some pro shoots they use a ton of lights, reflectors, scrims, diffusers, etc. (not to mention several assistants)...
Studio flash involve ways to alter two basic light properties. Light softness. Light direction.
Light softness comes from large sources. So you must spread the light out, using softbox, umbrella, reflector, or simply bounce flash light against the wall/ceiling..
Straight-on light direction looks boring. Altering light direction requires the flash to get off the hot shoe, or bounce the flash so that light got reflected back from the wall/ceiling/reflector. Your 580EX already has wireless capability, so you can take advantage of it by buying a slave flash, like 430EX, 420EX, Sigma DG 500..
There are some known good combinations of light softness and light directions for portraits. They have different looks. Learn some basics, like key/fill configuration, light ratio, flat/short/broad/butterfly lighting, etc. Then buy the equipments to acheive the look you want...
You might take a look athttp://www.strobist.com - an interesting alternative to big studio lights if you are willing to learn a little bit...
Yes, point the flash head to the left most of the way around, and slightly up. That gives you diffuse shadows, but not a completely flat look. You need white or off-white walls and ceiling for this to work right...
Might take a look at the Gary Fong lightsphere stuff. It's not as magical as his marketing videos would lead you to believe IMO, and it's pretty expensive for a piece of clear rubber, but it does improve on camera flash considerably...