depression - what is the cause, what can be done?

Well there are many causes of depression, some say it's genetic while others say it environment and it could even be a combination of the two. I've suffered from depression for 6 years and have been on zoloft which seems to help a little, but I find as time goes by it seems to work less and less. Apprently exercise and staying active somehow is supposed to help, even volunteering somewhere.

thinking much about the things taking loads of pressure and not doing effecient work leads to depression

Depression can be caused by alot of factors. You can do alot of internet research on that. As far as what can be done...exercise, quiet time, medication. For me, just saying to hell with it and getting drunk always seems to help.