I can only tell you about how I'm trying to maintain mindfullness. By practicing mindfulness of breathing (ānāpānasati) every day, sometimes with using the mantra "Buddho" sometimes without, depending on how mindful I am, thus helps me a lot to remember that I "want" to maintain mindfulness the whole day long. So when I remember that I start saying "Bud-" in my mind at the in-breath and "-dho" at the out-breath by whatever I'm doing at the moment. The more I do that, the more often I remember to maintain mindfulness throughout the day. I try doing it particularly when I'm doing sitting-meditation but also try to remember that while standing, walking or lying and whatever I'm doing. I would say it may be a sort of "self-conditioning" which leads to an automatism in maintaining mindfulness.
And in times when I notice that I'm not very mindful at all, I try to see it with wisdom. Everything is impermanent and I'm (still) a so there are times when there is more mindfulness and there are times when there is less. No reason to be upset, but important to be at least simply aware of.
Maybe it's useful for you...
Thag 1.20. Ajita - I do not fear death; nor do I long for life. I’ll lay down this body, aware and mindful.