I think once a day in detail is probably too much and not healthy.
I think 'often' isn't necessarily unhealthy, so long as the thinking make you live life more.
If you're sat in the house, turning down opportunities for living, allowing relationships to slide, then it's not good for you.
The bottom line is of course, you will die, you cannot really choose who or when you do (unless you really believe there's nothing more worth living for and are able to end things) you can hope it's quick, or quiet, you can only prepare your children to cope with their life beyond you (and you should do that whilst living certainly!).
The best preparation for death is life.
I don't think you can simply 'put it out of your mind' BUT I have found that if I follow through the wondering I act. So, how will my children cope without me? I follow through and then ask, how can I prepare them now for the future?
I always wear clean knickers.
But my biggest fear is dying slowly, so slow I don't even notice, but if that's for me I can't change it. I do have a lot of very open conversations with family (my parents, siblings, DH... not the children just yet) about 'what comes next' and feel like we've covered most angles.
I write a lot. Re-read and delete, noting important musings. I have ONE PAGE of thoughts editted from 20 plus years of musings. I aim to leave only 2 at most!