Have you a list, even if your tasks are just 'Read X paper'. I used to write lists on the disposable benchcoat on my bench in the lab each day. I found having it in front of me kept me focussed - when I started to think I had nothing to do, the list reminded me there is always plenty to do. Other students in the lab started writing their own lists on their benchcoat too!
Even now that I have finished my labwork and thesis, I still write lists most days. Todays:
1. Pick up my partners suit from the dry cleaner.
2. Apply for the job I heard about yesterday.
3. Buy a gift for a christening we are going to on Sunday.
4. Buy a belated birthday present for a friend and post it.
5. Phone my sister back.
6. Go to work at 5.30pm.
7. Book car in for a service.
This is ridiculously unnecessary because I would remember to do these things even if I didn't write them in a list, but I just like lists. Maybe lists will help you stay focussed too?