"Ooh, what a snug and shiny box it ships in... and what a desirable curvaceous phone... The UI is soo snappy and 'swipey-feely' and lovely... OMG, apps!! Must... download... everything... I can't bring myself to exit the marketplace, somebody STOP me! ... Oh, better plug the charger in... Ooh, location-based ferret-burger locator, who woulda thought? ... Actually, you know, the battery life is really a bit naff..."
Having installed probably most of the estimated 80,000 apps available, many of which will use battery-sapping features such as GPS and wi-fi, you may well find yourself searching for a mains socket or a USB connection rather frequently. Even with the default, out-of-the-shiny-box configuration, you might well find that you need to recharge more than once a day (depending on your phone).
Luckily, there are some immediate, and pretty straightforward, steps you can take to extend your Android phone's battery life. HowToGeek have put together a decent checklist to this end. Some of it's obvious - switch off GPS and wireless, of course - but there are some surprises in there too; e.g. use Android's built-in battery usage screen to find out exactly which apps have been sapping your juice. Another useful tip is to reduce the screen's brightness, and reduce the frequency with which certain apps poll for data updates.
It's also well worth putting the Powerwidget on your home screen, so you can quickly switch wireless/bluetooth/network services on and off without burrowing into the settings each time.
For the full battery-saving checklist, head over to the Geek.