Surround yourself with positivity, just shrug off the negative, try not to do anything (anything at all) that makes you feel bad emotionally, experience new things once in a while, and lastly and most importantly, commit to it!
To me, thinking positive is more of a lifestyle change then a thought process...
But that's just my advice, one of many ways to do things.
Its hard. I'm just changing every negative thought I have to a positive one, even if I don't believe what I'm making my mind think. I'm trying to train myself out of thinking negatively. You have to start somewhere and just try anything, it's a long hard road.
You'd be surprised at how you get used to swapping your thinking around, I never thought it would work either. I'm right at the bottom now, I've lost everything that matters to me, yet I'm okay because I don't let myself think or say anything negative, even though I do slip at times and be negative that's okay too I just pick myself up and keep going again.
And for me the forcing myself to think positive gives me more determination to change my lifestyle and move forward.
Yah like the first responder said, surround yourself with positivity.
It is known that when you or I, or just ANYONE, is being negative - this gives off quite a negative vibe to others which makes THEM feel down as well. I'm sure many of us despise it when that happens.
So surround yourself with positive people, and you will see (without maybe even noticing) how it has an effect on you as well (turning you into being more positive).
Obviously it doesn't always come easy.. sometimes negative thoughts take control as well :\ Which totally sucks. But it does take a bit of discipline of the mind in a sense to change your lifestyle to positive.
Little things count. Trying something new and staying committed sure helps...