Why not offer $25 worth of free Aps for a new sign up? Everyone loves Aps on their phones. It can be a simple.gift card for the Ap store through which ever device they have, for example an iPhone or a Samsung.
If they get 1-2 GB data plan, $25 and $50 worth of Aps if they choose a higher plan. I know I would have liked that.gift from you for signing.
I already like your plans now and am going to upgrade next month to more data. I would greatly appreciate some incentive for doing so even though your plans are already competitive. People think they are getting something of real value when there is an free.gift with a purchase.
Or a free case for your phone with a value of $20 when you get a new phone? Or even just a $20 value card for buying something of your choice for an accessory for your phone.
I hope you like my ideas.