This is completely true. Sometimes you have to wait for an 'up' day to get things rolling a little bit, or if you have a friend who is supportive don't ask them to come talk, ask them to help you make it out of the house, or pay someone $40 for some cleaning help so you aren't sitting in piles of crap. It's knowing what in theory will help you get better and then using opportunities to get there.
I suggest writing a list, based on experience, of things that do help with your mood. It helps me remember when I'm down, and I completely forget everything that makes me feel human - from taking a bath to have a decent meal. Right now, I'm feeling pulled down the slide to depression, but I also know from experience that exercise can help slow that down and also make it more shallow, so I'm using the time to stock up on some healthy food, get little chores done etc. and basically try to manage things while I've still got the energy. Living with depression isn't 'just' doing any one thing, but there are a number of strategies that can help. Good luck!