Here's what works for me:

1. Break your work down into small tasks which are (1) defined in scope (2) short - e.g. 2 hours.
2. Start the day by writing them down on a paper, in order. Draw some lines through - stuff you expect to get done before lunch, stuff you'll get done by end of day, etc.
3. Work your list, crossing items off as you go.
4. Time box things - if something's starting to drag, give yourself a time limit to research before you ask for help, or solve in a simpler manner.
5. If possible, structure your work so that you're publicly committing to these timeframes - entering estimates in bug tracking, etc.
6. If you catch yourself killing time researching, reading, etc., then invert the order - for example, allow yourself a 10 minute reward if you successfully complete a 1 hour task on schedule.

I've seen several comments that you should spend less time on Stack Overflow. If you're using it right, Stack Overflow should make you more efficient, not less. Steer clear of discussions and focus on using it to get work done.