Even those of you with an iPhone or iPad that has been grandfathered for your unlimited plan, you might want to take note. If you decide to enable tethering on your iPhone, your unlimited plan goes away. You have to switch to the $25 2GB data plan. Your laptop hooked up to your iPhone over a 2GB connection will go through 2GB before you know it if you are frequently doing crazy things like downloading music or watching videos.
LifeHacker has an excellent walk through on a number of tools you can use if you have an iPhone or Android based device, as well as a few tools for your laptop running Windows, Mac OSX or Linux. I may have to download the laptop tools to track my usage on my 5GB limited MiFi connection. Having worked with a computer that has the Verizon built tool to connect and track usage, I quickly decided the MiFi was for me so I wouldn't have to install Verizon's software, but I am flying blind on my monthly usage.
As a last resort, LifeHacker also has info on web based tools your carrier may provide. That may be more useful if you have a device like the MiFi and use multiple computers with it.
Keep in mind that tracking tools can work both ways. You can keep yourself from running over your limits, but you might also find you have over-purchased on your data plan and could save some money by dropping down to the next level.