Outdoors for fill, it's direct flash. Forkball is right on the money here. Besides, when you're competing with the sun you need all the power you can get, and diffusers rob power.
Indoors with low ceilings, you have a plethora of economical choices. The usual idea is to throw most of the light against the ceiling to light up the general area while directing a little forward to brighten up your subject and fill shadows. I generally recommend the index card & rubber band approach, though my flash units are encircled with velcro so I have a homemade piece of white plastic with a velcro strip that I use. Recently I was using a new flash unit (no velcro) and I stuck a white Post-It note to the back side. Worked great at close range.
If you're in a room with high ceiling or outdoors at night, the only way to improve the light is to make the light source bigger. The Lumiquest Pocket Bouncer fits this bill (I use the Promax System with white insert, which is basically the same thing).