The first suggestion I have (which is what I usually do when feeling depressed too) is to force yourself to get out there and do something for someone or something else. Just go drag yourself out toward your curb with a trash bag and start picking up the trash all around the streets... or go talk to an elderly family member or neighbor and ask them if there's anything you could do to help them out a bit that afternoon... or (my favorite), give some of your time to the local animal shelter. (But sometimes they have a registration process and other red-tape one has to go through to volunteer their time there)... really, anything that gets you out of the house is good, but I've found that selflessly giving back is the best way to instantly feel better about oneself. Other ideas: movies... go to the movies, or pop in a DVD. A good bubblegum, brainless action flick or comedy is always a good way to lighten the spirits. (Endorphins are released in our brain when we laugh and they actually do, scientifically, increase the feeling of happiness.) Also, taking walks and paying attention to the nature in your surroundings is good; or drawing, painting, photography, etc. (I tend to stay away from writing though because it actually can make you feel more down than when you started... it's all "brain talk" on paper when doing writing, so it generally turns the depressed-direction pretty quickly when one's already feeling bad.)
Another thing to consider are hormones- female hormones play a very large roll in our feelings (among many other things)... if it's getting close to your menstruation time, that may explain it. If you have started a different herbal/vitamin/prescription medication regimen, that can also effect the hormones. And they can just go all weird for no reason too... so it could just be hormonal changes that are effecting your mood (and in that case, we never know why, but it is fixable- go get a blood test if you think that may be it.)
Lastly, there's herbal stuff like St. John's Wort that's supposed to help lift the spirits... do use caution though, if you also take any type of birth control pill, St. John's Wort has been proven to decrease the efficacy (effectiveness) of the birth control pills... so one can easily become pregnant when taking St. John's Wort in addition to BC pills if they're not using an additional means of protection as well.
Meditate on a daily basis. Take daily hot showers and visualise yourself being cleanses. You may do some cleansing as well. Getting some vitamin B is very recommended as well. Eath healthy food and improve your diet.
Here is a site you can check out, it is called "how i beat depressin" and i think it can help you a little: