I believe these eReaders use the most power when changing pages. So how long one lasts on a battery charge will be a function of how many pages you turn (how fast you read, and how frequently you read). I guess you could set your font size to really really small, therefore in theory you'd turn fewer pages, thus using less battery.
Also, are you using the light or not - that will certainly affect battery usage as well.
I agree with HarryT - the battery lasts plenty long for most peoples usage. Battery life (given how long current eReaders typically last) would not be a consideration in my purchase decision either.