For Christmas I got an iPhone 4 and I love the the thing so much. I really do not know how I survived without it. Then in March we got my husband the iPhone 3. He doesn't love his nearly as much as I love mine, I think he is nuts. Now unfortunately AT&T no longer has unlimited data plans so we have to be careful about our usage away from home. This isn't a problem for me since I am home on our WiFi the majority of the time.
My husband is part of a fraternal organization in town and it turns out that a few of them were emailing photos from their fundraiser event back and forth to each other. Each photo was about 5 MB so if you add up a dozen or more photos, there is where all of his data went. After we figured that out, now he knows that he cannot send large files back and forth from his phone. Or he just has to wait till he is home on WiFi to download them. Though I think it would be better for their organization to set up some type of online file sharing to that they all have one place to upload their documents and images to share with one another.
Sine we went through that I now monitor our data and minutes usage more closely. I email him updates weekly or as needed to let him know what he has left or if I see a sudden spike. I think he appreciates it - or at least he should because I am saving us money in overage charges. I don't care if my new nickname is the cell phone police.
Do you keep an eye on your cellphone usage for yourself, spouse and/or kids? I have to imagine you really need to watch it if your kids have phones. How do you make sure no one goes over their limit?