While listening to a recent press conference given by President-Elect Obama, I was struck by how many times he hesitated slightly before he continued. (I don't think this is an indication that he's not sure by the way; I rather think it's because he likes to pause and consider what he is saying before he actually says it.) The way Mr. Obama hesitates is to say 'aah' or 'uuh' usually.
Let's see the ways you can hesitate in Japanese.
えーと (eh-to) and んーと (nn-to) are fairly neutral. They are sort of equivalent to the 'aah' or 'umm' or 'uhh', sort of unconscious pauses. あのー (ano-) may indicate a bit more deliberate hesitation, though the difference is subtle. Some Japanese people even insert some えーと etc. when they are speaking English or another foreign language. You can hear that from the Japanese woman speaking in this BBC broadcast about about a Shinto exhibition (RealPlayer required).
Next time you listen to a native Japanese speaker, see if you can pick up their えーと んーと and あのー.