In experiments with students it has been found that writing down your thoughts, in your own handwriting, can help you feel more positive, provided you fling away your negative ruminations and keep your positive ones close.
Professor Richard Petty of Ohio State University Psychology department collaborated with colleagues in Spain and tested 83 high school students.
Spending time looking at your negative thoughts make you feel bad about yourself. Throwing out negative and positive thoughts immediately has little impact on you, but putting your positive thoughts in your pocket or purse and referring to them later, has all round positive effects on your mood and future behaviour.
Computerised lists that were either retained or deleted had some effect too, but simply imagining that you had deleted them didn’t work.
(Reported in Human Givens Volume 1 2013 from Brinol P et al, Treating thoughts as material objects can increase or decrease their impact on evaluation. Psychological Science, 24, 1, 41-7)
My comments: this little tip could be very helpful. I know that people who keep journals tend to be more depressed than average. This could be partly due to the introspective nature of journal writing but also perhaps because negative thoughts or events can be reinforced by referring to them or even just carrying them around!
For avid diary writers perhaps they should keep two journals, one only keep the good events thoughts and another much smaller book that can be thrown in the trash every so often, preferably quite frequently.
It could also help when you want to achieve something. Put all of the pros in one list, all the cons on the other, and simply toss out the cons!