This is my favorite time management principle. Even though it's hardly about saving time per se, but still... Don't be afraid to have fun! It's OK to have fun. Don't feel guilty for having it. Having fun is not losing time. So go ahead, grab your snowboard and go to the mountains for a week. Have a dinner with your wife. Take your kids to the waterpark. Try skydiving. Have a motorcycle trip across a couple of states (I did this two months ago on a rented bike, and it was amazing). Play a video game. Heck, write some code, if that's fun to you!
Have fun. It's fine. Don't put it off for the "better days" - there's no such thing as "better days" when all the work is done and the goals are achieved. By that time your body might not be capable of having fun anymore. You're getting old, remember? Human intelligence peaks at age 26 and keeps declining afterwards. We all get dumber over time. So go out and have fun right now before it's too late.
"The most dangerous way to lose time is not to spend it having fun, but to spend it doing fake work."is my favorite quote by Paul Graham.
Now, the actual hacks:
The "unsubscribe" button
Unsubscribe from all the newsletters and SMS-ads. Instead of just deleting an annoying message - take a second to click the "unsubscribe" link at the bottom and save yourself hours in the future. Unsubscribe from all those "Follow-suggestions", "Quora-digests", "Here's what's happening on Twitter" etc. etc. These emails seem so useful at first... They're not.
Turn off the "passcode" on your phone
Turn off the "passcode" when unlocking your phone. These little things add up, you know... If you're concerned what happens when you lose your phone - there's a "remote wipe" feature for both Android and iPhone (via the "find my iPhone" app). Also, there's the "sign out all my sessions" button in Gmail and many other apps.
Use a hands-free
This one looks obvious but I was surprised how much more you can do with both hands available. No need to buy a fancy bluetooth headset, simple handphones that came with your phone will do.
Set iPhone in "Do Not Disturb" mode
OSX Mountain Lion "notification center" also has this feature, by the way.
Take a break when you're stuck
..and get back to that task later. Most of us have a really short attention span and our productivity degrades after a couple of hours of working on the same task. Take a break, grab a coffee, chat with someone, go for a walk. Then get back to it.
Set you phone email to "manual check"
Oh, this one saves a ton of time. Email can be a productivity killer. Set aside 10 minutes every hour for email (30 minutes every 3 hours is even better). Only few emails require immediate attention, and scheduling your email work can be a tremendous drain on productivity.
Don't waste your "on-desk" time
Workspace is for work. When you're behind your desk - get the work done. Check emails and Facebook on the go, use Instapaper or another "do-it-later" service for interesting but distracting websites.
Learn to say "no" to your friends
"Hey man, you know someone who can help me with XXX?" "Hey man, I'm thinking of launching a web-service, would love if someone technical glanced at my plan"
I get a ton of these... Don't get angry, your friends might not know, what it's like to be a founder. Instead of getting angry - find a polite way to say "no". Since most of the time you're on the road anyway, a short "I'm on my way to the airport" or "I'll be out of the country for the next week" gives them an idea about your way of life. They'll understand.
Have a short list of "things to do today" every morning
Share your hack in the comments!