with camera-mounted bounce flash -
when you have anything other than a light-coloured, opaque, flat and low ceiling, you have to be constantly aware of the direction of your bounce flash - pointing it straight up or slightly forward will not be enough.
before firing, take a good look at where your flash head is pointing. remember that light travels in straight lines - if it hits a reflective surface at an angle, it will bounce off that surface at a similar angle. which part of the ceiling will the first bounce hit? what direction will the light be reflected towards? where will most of that reflected light land?
once you bounce off to the side (relative to the shot), especially if you are using it as a main light, the scene may end up being unevenly lit. this is most apparent when using wider angles - the shadowed area may look very strange. you may be able to deal with this by not including so much of the environment (or the floor) in the shot.
sometimes direct flash may be your only option. the 'bright subject dark background' effect can be mitigated by underexposing the ambient by 1-2 stops, and avoiding showing too much of the floor or walls leading up to your subject in the scene.