There isn't a lot you can do with it other than spend it on calls, texts and data. You can occasionally run into online services (usually digital download purchases like apps, music and films from the Google Play Store but some places accept it in exchange for physical goods) that can be charged to your phone. edit: did some googling, you can apparently also use it to donate to charities or enter paid competitions via text or bet on things but the latter is likely dependent on what, if anything, is legal and available in your region.
Regardless, I can't speak for your provider, but at least in my case I don't have to keep topping up to keep my phone active...the credit I've added may expire but I can still receive calls for six months after without recharging so I'd only have to add more credit if I needed to use calls, texts or data so often times I go a few days after expiry before I have any reason to recharge. Yours may not work like that but it's probably worth looking into. I also have the option of credit taking six months before it expires but if I take that option I don't get as many 'free' calls, texts and megabytes so since I use quite a bit of data it's not the best choice. If your phone becomes completely unusable the moment your credit expires I suggest looking for an alternate provider that better caters for lower usage (and maybe has better coverage).
