Are you using any peripherals, and if so, are you using a USB hub with them? Once I get your answer I can comment better on a supply.
In general, a 5V 1Amp mini-USB supply works well enough. I'm running a pretty heavy project, so I juice the Pi from a powered hub connected to the Pi using a 5V 2.1Amp DC supply (I'm running WiFi stick, keyboard and mouse, and a bunch of GPIO pins). Hope this helps