A quick way to diffuse the light a bit, when there is no available surface to reflect the light, is to extend the bounce card, and raise the flash so it does not directly impact the subject. A 45-degree angle is often a good place to start. I will typically set the camera in Manual mode, aperture f/5.6, and a shutter speed of 1/100 or so, with the flash on E-TTL, with the head extended upward at a 45-degree angle, bounce card extended, as a starting point. Often, the result is quite good, with no flash exposure compensation adjustments needed, depending upon what I am trying to achieve.
Flash Exposure Compensation for the 430EX II (and 580EX II and 600EX-RT) can be easily adjusted on your camera's LCD, assuming the camera is a 2007 or later model. The flashgun's menu is accessed through the camera menu. To make this quicker, I set the flash menu to appear in my 7D cameras' "My Menu settings," which is the far right box in the 7D menu. This will vary a bit with the various Canon models. Notably, the original 580EX and 430EX, as far as I know, will not work this way with the camera menu. The ST-E2 must also be controlled directly, not with the camera's menu, though its radio-technology successor can be controlled with the camera menu.
In Aperture Value mode, use of flash is trickier, as the camera tries to use the flash only for fill-flash. I have little experience using flash on live subjects in Av mode, as I generally set the camera in Manual mode when I install the flashgun or ST-E2.
Canon 5Ds R/7D2/7D/5D/40D/1D2N; Nikon F6/D3s/D700/FM3A/Coolpix A. Lens selection undergoing changes; some favorites: Zeiss 2/135 APO Sonnar, Canon 100mm f/2.8L Macro IS and 135L, Nikkor 14-24/2.8G and 200/2 VR.