If you're in bright light outdoors, instead of using on-camera fill flash, consider using a reflector. It can be something as simple as an uncut sheet of mat board / foamcore / etc. Even better, get one of those silvery window reflectors you put over your dashboard on a hot day, and you've got something that folds up nicely -- and probably even in your choice of silver or gold (for a warming effect). You'll automatically get a good fill ratio with perfectly balanced color rendition, plus it'll be coming from a much larger (and therefore softer) source than the pinpoint of the flash.
Depending on the shooting location, you can even do without either by strategic placement of the subject and the sun and your surroundings. Look for pavement or (neutral-colored) walls to act as your reflector, as well as shade for the model. Keep in mind that the kinds of light levels you're talking about will make your model want to squint, so you might even want to get a translucent white umbrella (or the like) to put some shade on the model and, again, soften out the light and reduce the contrast of the scene.
The next thing you want to do is put as much distance as you can between your model and the background. With a 50 on a 1.6x crop body, if your model can touch the background, it's going to only be slightly out of focus. If your subject can take a step and touch the background, that's still probably not what you're looking for. Ten feet should start to get into the not-bad range. You're not going to ever have too much distance between your model and the background.
If even that's not enough, the answer is to start spending money. A longer focal length with an equal or bigger pupil will do the trick, with the ultimate in the Canon lineup being the 600 f/4 (and the 400 f/2.8 right behind). You'll need a megaphone or a walkie-talkie for your model to hear you, though, and you'll need a mule to haul the lens. And a five-figure credit limit to buy it. But also consider a "sleeper" lens like the 100 f/2 -- inexpensive, damned good quality, and a much shallower depth of field (once you back up to get the same framing) as your 50 f/1.2.
A larger format with an equivalent field-of-view lens will do the trick, too; an 85 f/1.8 on a 5D will have lots shallower depth of field than your 50 f/1.2 (or even the 100 f/2) on your 50D, even though the framing will be basically identical. And a 135 f/2.8 on medium format will be shallower still, again with the same framing. You could almost trade your 50D + 50 f/1.2 for a classic 5D and an 85 f/1.8, but a medium format system will cost more than the Canon supertelephoto.
Cheers,
b&
so with a 50 1.2 you shouldn't have any problems to get a shallow dept of field. It sounds like you use some kind of automatic mode and the camera is doing whatever it wants. Try using full manual mode (ALSO MANUAL FLASH!). 
