An almost universal complaint of clients in the city is that don't have as much energy as they'd like/used to have. Now whilst a lot of us probably think that's part and parcel of getting old, I personally think it doesn't need to be the case.
I've blogged fairly recently on how certain foods can rob you of energy (http://nutritionistinthecity.blogspot.com/2011/03/energy-thieves.html), but what we also neglect to do is get sufficient rest. Yes this includes sleep, but also just sitting down and doing nothing - something most A type, multi-tasking city folk find very difficult.
It seems as if in learning to operate at hyper-speed and become super efficient at work that we unlearn how to relax properly. Watching tv, going to the gym, playing squash, might all feel like stress relief but infact they still provoke a stress response and don't count as true relaxation.
What I highly recommend is that when you get home from work and just feel like collapsing in front of the tv, that instead you spend 10 minutes sat somewhere comfy, or even lying down, and just do nothing - no reading, no talking, no watching tv. Try and keep your mind calm too - let your thoughts drift around but don't start planning what you're going to do that evening or replaying about what happened at work that day.
You may fear you'll fall asleep, and this may happen, but what you'll be surprised to find is that after even just 5 minutes you're energy levels will come back up and you'll feel refreshed and ready to do something more constructive than watch re-runs of Jersey Shore!
If you really struggle to switch off mentally I recommend reading The Power of Now
to help you get into the habit of clearing your mind.
Doing nothing (for a short period!) is also good for weight loss reducing your circulating levels of stress hormones which encourage fat storage, particularly around the middle, so if you're trying to lose weight it's worth making some time to do nothing at all!