From a less technical perspective you should at the minimum have anti-virus software running on their local machines. You should also consider having a VPN (private network) for them to login to do work functions if you and/or your staff are working on items that could be considered "sensitive". Also, hard-drive disk encryption has become pretty common. This helps protect from threats of items are lost or stolen. And as always tell them to a secure password and have a standard in place to enforce such a policy like password:

• Must be at least 8 Characters long
• Must have upper and lower case numbers
• Must have at least 1 number
• Must not be the same as your username

Stuff like that will definitely help.

Agree 100% with Jef regarding antivirus. Personal smartphones and tablets which access work email should ALWAYS have an unlock password as well as the ability to remotely wipe the device if it gets lost.We use Office 365 which allows you to easily do this and has the added benefit of selective wipe, which allows you to wipe only business data from the phone leaving personal items such as photos alone.