While we're on the subject of why Sprint is the carrier to go with nowadays ($50 contract extension credit "just because," unlimited data without expensive and complicated tiers **cough Verizon cough**, great customer service, etc.), I wanted to throw yet another reason for existing Sprint customers to stick around.
Side-by-side with the $50 credit promo that I mentioned above, it turns out that the company is running a different deal in parallel that pays you for a month of free service right back to your account upon contract renewal for 2 more years. It does not affect your phone upgrade eligibility at all and is a very effective retention tool, so you can ask for it now, even if your contract still hasn't expired, get the contract renewed, then grab a new phone later). This promotion works only for primary lines, so if you're part of a family plan, only the main phone number can benefit (in contrast, the $50 credit works for any line).
Tipped by Jason in the $50 credit post, I called up Sprint to renew the contract on one of my lines and asked them about the free month instead of the $50. The CSR knew exactly what I was asking about, though she wasn't sure just how long this deal is available for.
If you're near or past your contract expiration, give Sprint a call (*2 from your handset or 1-888-211-4727 from any phone) and ask about this promotion - it can save you quite a bit of cashish.
Note: The credit applies only to the basic cost of your plan and doesn't include any fees or add-ons. For example, my $129.99 family plan ends up costing me about $170 after all add-ons. For me, only $129.99 would be refunded.
Let us know how much you end up saving and good luck!