If you have a monthly limit on how much data you can use, here are some tips on what types of phone use will gobble up megabytes:
Streaming video and videoconferencing. One minute of YouTube-quality video eats up 2 megabytes. Unless you're on WiFi, if your plan gives you 200 megabytes per month, you can't even watch Lady Gaga's "Telephone" video once a day. AT&T charges $15 a month for 200 megabytes, though Verizon is not offering such a limited plan.
Streaming audio. Audio consumes about a quarter of the data that video does, but 10 minutes a day will break the bank if you're on a 200-megabyte plan. One hour a day of Pandora consumes nearly a gigabyte, which you can afford if you're on a 2-gigabyte plan and don't use other data-hogging apps.
Photos. If you're a real shutterbug, photos can consume significant amounts of data. Sending and viewing photos both count toward your monthly limit. Posting 10 photos per day eats up most of a 200-megabyte plan. If you're on a 2-gigabyte plan, you probably don't have to worry about photos.
Maps. Navigation apps consume lots of data when they retrieve map images, up to a megabyte a minute. You're also likely to use them for long periods of time when you're away from WiFi, such as when you're driving. Watch out for these.
Web surfing. Roughly speaking, 10 Web pages a day will eat up about half of a 200-megabyte plan. Again, those on 2-gigabyte plans don't need to worry much about surfing.
Facebook. Roughly equivalent to Web surfing. Status updates won't take much data, but sending photos and viewing friends' pictures will.
E-mail. Most e-mails are tiny, in terms of data. Basically, you can send and receive e-mail messages all you want, as long as they don't have attachments such as photos.
Twitter. Like e-mail, these short messages don't use much data, but if you follow a lot of people and click on links, usage adds up.