It’s true that it’s hard to remember keeping a smile on your face when you go through a normal day, but what I find helps is looking at others and noticing their facial features on buses, trains, on the street etc. Once I realise they have sad/indifferent expressions it usually reminds me of my own facial expression and I remember I should smile more. But as a warning it’s not good to make eye contact with a sullen looking person sitting across from you and then suddenly grinning for no apparent reason… it’s been tried by me and it was extremely awkward.