Hollywood movies often show secret agents tossing cell phones out of car windows, and grabbing new ones to activate. In today's world of almost universal surveillance and tracking, that is actually fairly good tradecraft. When operating in guerrilla warfare mode, a cell phone that is used more than a few times is a liability. So is a cell phone that is "turned off", but that still has its battery installed. (They can still be tracked.)
In summary, here is some cellular phone tradecraft for times of genuinely deep drama:
1.) Don't create a paper trail when buying clandestine phones. Pay cash for cell phones and don't give your name. Preferably buy them in small stores without video surveillance.
2.) Activate phones only as needed.
3.) Never "recharge" the minutes on disposable cell phones. (This leaves a paper trail--at least leading to the place where you bought a recharge "minutes" card. And buying minutes via a phone call and credit card transaction leaves a huge paper trail.)
4.) Set a "phone talk time limit" for your group, depending on the then-current severity of the threat. Once you've reached the limit for each phone discard it. (But save the batteries, if they interchange.)
5.) Never program any cell phone numbers into your phone.
6.) Also carry a retained "cover" phone, on which only totally mundane (non-operational) calls are made. If you can make your operational phone disappear, then your cover phone will give you some plausible denial. (But you won't be Teflon Coated, since the geographical movements of your cover phone can be correlated to operational events or calls from any of your clandestine phones.
7.) Discard phones discreetly, with the batteries removed. Alternatively, you can leave the battery in if you want to lay a trail to confuse those pursuing and you suspect that phone location is being tracked.. (You can mail the phone to a random address that is a thousand miles away. (Use a padded envelope and just drop it in a mail box.) Or you can leave it in a donation box for regional charity. (These charities usually send donated items to a sorting center.)
8.) Keep in mind that cell phone Subscriber Identity Module (SIM) chips are quite compact and can be moved from phone to phone.
Take a look at the history of how Ryan Fogle was bounced out of Russia. He used some very bad tradecraft. Learn from the mistakes of others.
One final tip: Reader Jeff H. mentioned that Tracfone now sell LG800G with 1,200 minutes loaded. The nice thing about these is that their minutes never expire. So this sort of phone would be a great phone to buy and just "tuck away for a rainy day."