I wouldn't like to just do one story or one type of stories all the time.

The advice would be the same for any kind of fiction. Keep writing, and keep sending things out, not to friends and relatives, but to people who have the power to buy. A lot of additional, useful tips could be added, but this is fundamental.

I started writing seriously about 1960, at the fairly advanced age of 30.

Mysteries I read for fun, so I will probably never write one, for fear of spoiling the fun.

And what we know, or think we know, about the universe of space and time is changing very quickly.

I wouldn't like to just do one story or one type of stories all the time.

If people ask me for the ingredients of success, I say one is talent, two is stubbornness or determination, and third is sheer luck. You have to have two out of the three. Any two will probably do.