Unbeknown to most, there are many health-related issues when it comes to surfing the net.
Increase your mobility: Invest in a mobile handset that is Internet and email-enabled. This would mean the luxury of being able to nip off to the gym in the middle of the day safe in the knowledge that you can deal with any urgent emails.
Do your thinking outdoors: Because much of what we do at work today is intellectual compared to physical, there's not much point staring at your computer screen straining your eyes trying to come up with the next campaign idea when you could be doing exactly the same thing outdoors. So if your fingers are lying idle while your brain is active, go for a brisk walk and be inspired.
Work is work, play is play: The availability of social networking tools and instant messaging means that too many people are spending extended periods at their desks engaged in a mix of work-related and personal communication at the same time. Resist the temptation and set your IM channel, for example, to 'busy' or 'unavailable' during the work day so that you are able to complete your work within the allotted time. Then, briefly after work, devote a specific period of time to social networking and then call it quits so you can get to the gym. Your physical health will improve and your mental space will be less cluttered.