I used to give my thoughts a free reign and just think about whatever popped into my head. This was fine when it was happy thoughts, but sometimes negative ones used to take over and bring my mood down. But then, thanks to broadcaster Chris Evans, I learnt that it was possible to control what you think about. And overnight my life changed for the better.
Think about your thoughts as different TV channels. So in my head I usually have a number of different channels available, some positive like fashion, designing, holidays, and some negative: like worrying about health and things that might go wrong. I used to spend alot of time worrying about the latter and I used to brood on it and bring myself down. But now I choose to change my thought channel, and be in control of what I think about. So if i'm thinking about something that is making me unhappy, I just switch to thinking about something else, like switching the channel on a TV.
This is particularly important at night time, when one wakes and starts worrying about things, which then usually means doing down a black hole of worry and being awake for hours and then being exhausted the next day. This night time worrying never actually achieves anything. So now if I wake in the night, I just turn my thoughts off, like turning off the TV.
It does really work, but takes a little bit of practice. I know that we all have to think about unpleasant things sometimes for practical reasons, but just focus on them for as short a time as possible or until you have worked out a solution, but don't brood on them for hours...