The easiest way is to use the slide bar and turn it off and on. That requires effort to remember to turn it off and on. If you don't like that, you can use Tasker. It will use trigger events like location (work) to perform a task (turn off wifi). The trial version is about 30 days to see if you like it. I had it then went Motorola and used the smart actions. Now that I have the GS3, I will probably resurrect the tasker.