When researching niches I think "what do people need or want that they can find out about it on the internet". It's seriously THAT generalized. I don't know a better way to explain it either.
Jon,
let me explain my approach for this.
generally, I do keyword research that reflects the behaviour of the visitor. So I used root keywords like
top
Find
best
how to
how do I
repair
loss
relief
save
for example, with the keyphrase "how to", i found the following keywords from overture:
how computer works
how to video
how to last longer in bed
how to get in shape
sure these terms have not much searches per month, but most of them are also low in competition and fairly easy to rank high in SE.
just my two cents.