Here is the recipe I have been using for the past 10 years that allows me to average under $10/month on my cell phone bill. Most months I average around $5/month. What do I mean by average? I fill up my T-Mobile SIM card 4 times a year or so. The pricing structure is $0.10/minute, $0.10/outgoing SMS, $0.05/incoming SMS on T-Mobile Pay As You Go. The balance lasts for the whole year, once you have spent $100: Pay As You Go FAQ.
I made the switch to a Samsung Vibrant running CyanogenMod 1.5 years ago from the first ever flip-phone I acquired, which has made making/receiving free calls and free texts that much easier; see Hangouts Dialer and email over Wi-Fi. This is my secret to not using up my balance, I never actually make calls with my T-Mobile balance, but instead use Google Voice. I swapped to a smartphone for the camera, GPS, and TODO lists. Where I go in the mountains, a cell data plan would not do me any good, being unable to access it.
Like most people, I really do not receive any calls during the day. When I am home, I have my computer on, logged into Google Hangouts which notifies me of incoming calls, and allows me to make outgoing calls. Your cell phone balance will not get touched at all, if you pick up through Hangouts. I also do all my text messaging through email. When I am going to be away from Wi-Fi access points for awhile, I then turn on text forwarding to my phone.
As long as I continue to do all my calling and texting through Google Voice, none of my contacts know my actual cell phone number to inadvertently use up my cell phone balance. To them, it appears that an incoming call is made from my Google Voice number. Do note that to make outgoing calls without any data access, you will need to have called or texted the contact once before while connected to Wi-Fi, or else call your Google Voice number and follow the prompts to manually enter the number.