Sunrise clock.
SAD light for when you're up/working anywhere with crap light during the day.
Going outside every single day without fail. If it rains, wear a jacket and take an umbrella, but get outside.
Vitamin D.
Lots of vegetables, fewer carbs, lots of oily fish and lean meat if you aren't vegetarian, eggs if you are, high protein pulses if vegan. Oats are also good. And tons of water.
Get moving. Don't stay in bed with the curtains drawn, drag yourself up, windows open, get warm water on your face/body and do something.
If you have exercise to a pleasant place to exercise, take it. It'll make you more likely to sleep well and have the benefit of exercise induced endorphins - they even happen when you hate exercise (they do me). Light weights, swimming, yoga, rowing, whatever it is, you want to keep strong and mobile.
Set a time for the 'end of the day'. Lights down, blue screen filters, no baths in glaring overhead lights, no caffeine after 2pm - this tells your body when it's time to sleep and it's fine to go to sleep early, as long as you get decent sleep and get up in the morning. You're giving your body a noticeable day and clear night, rather than the dreary winter not-really-daytime-at-any-point routine.
Essentially, the most vital thing is routine, routine, routine. Get into one and even though it's hard, you're more likely to do things that help and you'll have more energy and motivation the next day.