Make sure you have the data rate set to 50Mbps and the gain turned down to 0 or below - this will give you a clean image with plenty of color depth. Using the live histogram will make it easier to avoid excessive blow-outs while filming. I've read that closing the aperture beyond f/8 can degrade image quality, too. If you know how to color-correct footage, it's worth experimenting with the cine gamma custom picture settings or importing customized ones that can be found online.
There are some nice settings that have been put out on the XF300 group on Vimeo by Brian Weed. I've downloaded them and done some testing. Also set my camera up with the BBC settings (there are two). They are also listed over there.