Deploying Apple devices in a small business

I'm the IT support for a small company that deploys IPads and IPhones to users that travel for work.
I want to point out that I am new with supporting Apple devices and am looking for advice with this.
We have had some change of hands of the devices, with people leaving and new people being hired. So this brings up the first problem of: Each person loads their own apple account to the devices. So when we get the devices back after they have left the company, we will reset the device and hand it off to the next in line.
Well, the second problem with that is that the previous user's account is still "linked" to the specific device and we are unable to "unlink" the account, well not easily anyhow, without jumping through lots of hoops with AT&T's apple support, which is who we get our devices from. We have been able to "unlink" one device of the previous user's device but it was a 4 week process of proving that the device did indeed belong to our company.
The main idea I am looking for here is, Does anyone know of a simple and cost effective way to manage what accounts, apps, ext. are added to the devices. OR A simple and tested way of easily unlinking the the previous user's account info from the device.