Something I've noticed in a lot of my clients and after a while even myself is the tendency towards negative thinking. Seeing the negative, looking ahead and imagining how things could go wrong, jumping to the worst possible outcomes, feeling depressed, feeling tired etc. Any of those sound familiar? This type of thinking is cyclical, meaning, we believe something is bad/bad will happen, we look for the bad things, we remember the bad things and therefore our beliefs are reinforced. How do we break this cycle? Look for the opposite. 
Sounds simple enough and it actually is but like any new habit it takes practice. If our automatic response is to see negative, it takes work to look for the positive. 
Now why is this important? 
1. "Happy thoughts and positive thinking, in general, support brain growth, as well as the generation and reinforcement of new synapses, especially in your prefrontal cortex (PFC), which serves as the integration center of all of your brain-mind functions." Sweet, we want our brains to grow, yes? It's very important to keep well as we age. 
2. Looking for the positive actually makes us more resilient, less likely to become depressed and overall have a better quality of life. Even when faced with adversity and stress, this type of thinking can help keep us going when the going gets tough. 
3. Positive thinking can help strengthen your immune system and make you less likely to become ill. Now this doesn't mean it's the cure for cancer, however if you're focusing on negative things all the time this can spike your cortisol levels which when this happens chronically can weaken your immune system. Therefore positive thoughts can help keep cortisol levels even and keep your immune system up. 
4. Positive begets positive. The moment you start looking for the positive you see more of it, this affects the way you see everything, the weather, your job etc. So for individuals suffering from low level Seasonal Affective Disorder or low level depression this can be quite helpful mitigating the negative symptoms they experience. A lot of depression can be chemically based and require medications but there is nothing wrong with adding this in with that. 

Okay now to the technical side: how?! Practice makes perfect
1. Write 3 positive things in your day planner/journal every day. Start with 3, guaranteed you'll be able to see more and more as the weeks go on. When I started my first week most of my things included naps, sleeping etc. It's branched out from there.
2. Use the buddy system. With your partner/bff at the end of the day tell each other 3 good things that happened that day. Ask them to call you out when you're being too negative.
3. Set reminders. A simple alarm going off on your phone reminding you to look for the positive can be very helpful when your day is becoming overwhelming.
4. If you don't have anything nice to say.....We've all heard this one. Now I'm not saying you can't vent about a stressful situation at work, etc, but if you keep bringing it up well that means you're just going over it over & over again. This isn't helpful, bringing it up to vent or get someone's opinion is healthy, stewing is not.

Good resources: rewire your brain, self talk

I've read the first, and the second comes highly recommended.