You just have to not have senior moments. I recommend that, before you lay an idea on the page in full form, you write a lot of it in your head. Do it over and over - phrasings and moments that convey certain emotions, etc. In this way you can form a notion of the personalities you are writing about and keep proper perspectives.

I've a major character in a series of stories who has a couple of very specific affectations. Because of those affectations he became more real to me and I was able to keep consistency with my story from moment to moment and story to story.