Print out a checklist
Printable CheckList is a good spot to print out a daily task list. It has been free and ad-free for many months, but I see recently they display Google ads at the bottom. But it is still free, and is great way to focus yourself on the next couple of hours.
Get off the Internet
The program Macfreedom will disable your internet connection for up to eight hours. It can be configured to still allow access to the local area network (like your printers and file shares), while denying access to the wider world.
Track your time
Harvest. You can print beautiful time sheets and reports, track multiple projects and tasks, and hourly billable time.
OK, now I really, really have to get back to work.