You are not alone, Rose. I think all PhD students experienced the lack of concentration all the time. Being into the 3rd year of my PhD study, I am no exception and have to constantly fight distractions.
A few things I have done recently to stay focus are:
1. Deactivate my facebook account - it took up too much of my time
2. Block certain people whom I used to chat very often at Skype - another source of distractions
3. Started a PhD blog - to destress whenever I feel the need of venting my frustrations
4. Muching melon/sunflower seeds while reading or writing - they relieve tension and are healthy source of minerals for my brain
Hope the above sharing could be of help to you.