It's tempting to start your day with easy tasks. Don't. Pushing a daunting task back is like holding a glass of water in an outstretched arm. At first nothing happens, but if you do it for hours or even days, you will soon feel the stress. Do the most annoying task first thing in the morning, and enjoy increased productivity and peace of mind for the rest of the day.
You make plans to go outside with friends. But at the last minute, it starts raining. What's your reaction? Some people upset and angry, and find the nearest person and start complaining to them. “It's not FAIR that it's raining. This _always_ happens to me.” That's not going to accomplish anything - the rain won't stop just because you throw a tantrum. The rain doesn't care. So make the best of the situation. What I do in such situation is go for a quick walk in the park (because rain has its own awesomeness), or just lie in bed reading a good Terry Pratchett book, listening to the rain beating on my window.
Just walk to a window, look outside, and then take a single deep breath, focusing only on that breath and nothing else in the whole world. This action sounds extremely simple, but you won't believe how much it can bring you a wonderful day.