I do mindful meditation, probably something like begginer's breath. I don't use any mantra. It's ok if thoughts go through your head now and again just don't pursue them, let them float by and keep returning to the breath.

Sometimes, I'll have a really great idea suddenly pop up in my head, but I don't analyse it, I just let it go and deal with it when I've finished. Ironically, I'd say the harder you find it, the more you need it.