I'm not the person you replied to but their list of strategies (gratitude practice, evoking joy, exercise) pretty much mirrors what I've been trying to employ.

> Saying it every day means little even if I write it down, and the writing itself feels more like a burden than a help.

Perhaps an obvious statement but our experience with any type of practice varies in infinite ways from moment to moment. At times things just click or maybe we've built up enough momentum that it could feel effortless, but on just as many occasions it can feel like wading through sludge. When it's the latter I have to ask myself just how am I showing up for the activity. How mindful am I? What's my intention? Perhaps most importantly, is the sense of gratitude actually being felt in my body?

If you don't mind self-help type books, 'Hardwiring Happiness' by Rick Hanson is a fairly accessible resource that stresses the importance of the somatic side of this type of work. The tl;dr is that if more parts of the mind pay _sustained_ attention to the embodied experience of gratitude, compassion, joy etc. then we're increasing the chances of training our minds. So if I find myself enumerating things in a journal that I believe I'm grateful for but the exercise feels contrived or flat then that's a sign I should either tune even more into large parts of the body (can be anywhere but for me it's usually my face, chest, and arms) or just attempt to evoke warm feelings in those areas. That last part can feel fake at times but there's probably value in learning how to encourage more mind processes to sign up for the practice. The OP alluded to this bit with "sometimes it can help to simply sit up straight and smile". If the body remembers what gratitude feels like then chances are that's going to influence the mind for the next few moments.

'Awakening Joy' by James Baraz is another book in this vein. In it the author makes the case that learning how to shift our baseline towards one coloured with joy and gratitude usually requires someone repeatedly and genuinely appreciating seemingly trivial things over the course of each day (food, shelter, mobility, pet, access to nature, etc.). Whereas shifts occurring solely due to significant positive life events are potentially less common.