It's a lot harder to get up off your ass than to hit alt-tab.
Harness that laziness to your advantage.
use your computer strictly to create stuff - code, write, design, photoshop etc etc. If you want you can install some software to block sites and apps on your computer.
This will reduce the distractions enormously. If you find yourself on ipad all the time you will feel guilty that you didnt create anything today and thus will be motivated to do something abt it.
If I need to concentrate, I physically unplug my notebook from the net (my only connection is ethernet cable) and move to a different room.
It works wonders. It makes me think hard whether it's really needed to check something on the web. Usually I just preload in tabs background info I may need later in the day to get the job done.
edit This is with all new components in the box; it's even cheaper if you can get "hand-me-down" components, which should be easy for anyone on HN.
Seriously, though, how do you make yourself sit down at the work computer?