Don't check emails, answer the phone, use instant messenger, other websites or the Betfair forum while the racing is on, this includes the slack period between races when you have 'greened up your position'.
That's the time where you can sit back for a few minutes while nothing much is happening and relax a bit, but you must still watch the price and be aware of what is happening.
It is very easy to get distracted and effect your trading. To really get in the groove you have to concentrate on every race, moving onto the next race when that one is due to start. Don't take your eyes from the screen except when nature calls. If you smoke then smoke in front of the computer or not at all, nipping out for a cigarette will cost you thousands of pounds over the course of a year.
The key is to concentrate for three and a half hours. You are a trader and nothing else, you'll be surprised how much better you trade when you don't allow any outside distractions of any kind, letting yourself be absorbed by what you are doing and really seeing the movements and imagining what they might do next.