This is what has worked for me:
- Make sure when you sit down to play you are comfortably seated in your chair.
- Make sure there are no distractions such as people, animals, skype, browser, television, even music can be a negative for some.
- Eating healthy and being on a good sleep schedule helps. Regular exercise improves the ability to focus too.
- Start out by playing short sessions. Maybe just 30 minutes one day. 30 minutes the next.
- Take an off day at least once a week, but don't be afraid to simply play every other day to start.
- Slowly increase the length of your sessions, though it is healthy to get up every hour or two, walk around, stretch out, stop looking at the computer screen and just refresh a bit.
If you're still having issues with focus, really try to pinpoint the reasons why you're having these issues, bump this thread and maybe we can give some more specific advice.