I start taking supplements - iron, Vit D, B complex. L-Tyrosine, Magnesium, Berocca sometimes
I try to increase exercise - online 10-15 minute body weight, yoga or kettlebell videos if I can't face going out, bootcamp and yoga flow if I can.
I stop watching and reading all the news and all crime dramas and novels. I switch to comedy films, series, TV recordings of good stand up shows, and feel-good movies. I play only really calming piano or mood music or upbeat songs.
I have a mantra - 'you don't have to want to, you just have to do it' about stuff like showering, teeth, clean clothes that look nice.
I make lists of what I have achieved each day, not to-do lists. They are very good at showing you that you may feel like you stared into space or went to bed all day but actually you got the DC up and dressed and fed and off to school, fed the pets, the birds, put a wash load on, watched a helpful online video about depression or did sme online CBT worksheets, had a shower, bought milk, collected Dc from school and fed them, bathed them, got them to bed. You realise you are contributing to the world in small ways even if it is just feeding wild birds and making sure your DC get to school and back and are fed.
I write gratitude journals. I know some people hate the, as they feel it's like saying: you have all this so you should be happy and grateful and not depressed. But for me it helps to say: even though I feel depressed I am glad of these small things happening in life.
I've learned to tell myself it will pass, it always does eventually, so meanwhile, look after yourself.