Smile - Wherever and whenever you can. The one thing in life that we should be infinitely generous in is in giving out our smiles. In addition to putting yourself in a positive state, you will also positively affect the people around you as well.
Read positive, motivational books or quotes - This is a fast and convenient way to put yourself in a positive state of mind.
Call a friend - Give an old friend a call and make it a point to talk only about positive things.
Get out of your usual environment - Go to the park or anywhere that is nice and peaceful. By being in a new or not frequently visited environment, you will not be affected by any negative association that you may have with a certain place, i.e. your office.
Work on your goal setting or review the goals that you have set.
Listen to uplifting and positive music - Well selected classical music or movie soundtracks will usually do the trick.
Read motivational and positive stories - For example, books like Chicken Soup For The Soul. Or just do a search on the internet for motivational stories.
Exercise - working out can encourage a positive state of mind. It helps to release stress and puts you in a heightened state of awareness which induces positive thinking.
Creative expression - creative expression through activities such as writing/journaling, drawing, painting, playing musical instruments, dancing, photography and others can encourage positive thinking. Take up lessons or regularly set aside some time for these activities.
Sing and dance along to your favorite song.
Enjoy a nice cup of afternoon tea or evening wine while putting on some soothing music in the background.