Did you know you can check your cell phone's voicemail from your home phone or another land line ? Just call your cell phone number, and when the voice mail picks up, press the star key (*). It will then prompt you for your password, and let you check your voice mail just like you are on the cell phone. If you spend a lot of time with voice mail, this could save you enough minutes to fit back in your plan.
Most carriers also have local numbers you can call for voice mail if you do not wish to use the simple method above:
T-Mobile: Replace the last four digits of your phone number with 9999, and dial that number. If your number is 603-555-1212, then you would dial 603-555-9999. Then, enter your cell phone number when prompted. When your greeting starts, press the star key (*).
Sprint: Similar to T-Mobile above, but replace the last four digits with 6245.
Verizon: Also similar, but use 8888.
Other Carriers: Try this handy list over at bridog.net
Also see our other cell phone tips.