Your best chance to get free airtime is probably to enter a airtime competition where you can win airtime...
You could also beg a friend to buy you some airtime or transfer airtime to you. Vodacom offers an airtime advance service which lets you use up to R10 of airtime when you run out - Your account will be debited with R11 next time you recharge though, so this is not free, but handy to know about if you are out of airtime and cash.
Otherwise speak to your provider about promotions that incentivise customers with airtime rewards and work the system! Examples include 8ta's 1 minute free for 3 minutes of incoming calls, Vodacom Color and Virgin's double your airtime on your first recharge promotion.