I just lock the screen (press power button once, then home button once). And again to unlock. If it's sitting on the table I just let it go to sleep by itself.
It doesn't use much power. It can stay in that mode for 9-10 days. You can even leave it connected to wi-fi, it'll keep for at least a couple of days still. This is how I usually leave it, because it'll keep receiving emails and google talk connections etc. (and play a little sound to let me know). Doesn't use much battery at all, it has a very efficient power saving mode; whenever the CPU goes idle (which is most of the time, as most applications are just sitting there, waiting for an event) the power drain drops by several orders of magnitude.