Those micro-USB jacks are very VERY fragile, and can be rendered useless with one good tug, like picking up your phone to take a call while it's still plugged in. Once damaged, you don't have much choice other than replacing the whole phone. That's what got me here in the first place, BTW. My old Motorola RAZR flip phone kept chugging along, but the micro-USB port finally wore out to the point that I couldn't get it to charge. My new phone has a Best Buy extended warranty that covers stuff like USB port damage.
Charging cradles can help, but they aren't a guarantee that your phone isn't going to be damaged unless you put it in the cradle just so. Also, some cables work better than others. If in doubt, try another cable.
To try the connection, gently apply pressure on the plug in different directions, and see if the charge light comes on. If that doesn't work, get a different (new is best) cable and see if the phone charges on that. If not, repeat the gentle probing to see if you can find a position that makes the connection. If it still doesn't work, take it to a Sprint store and see if they can do it.
If nothing works, your micro-USB jack is probably toast. Bite the bullet and do what needs to be done.
