How can I tactfully deter a houseguest who wants to arrive too early?

If she knows she can just 'let herself in' whenever she visits, she will continue to show up practically unannounced. I would have a casual conversation with her and let her know that it makes you feel guilty when she shows up as she does because, as you are friends and do not see each other as often as you used to, you would like to spend more time with her on some of her visits to catch up on what is going on with each of you and ask if she could give you more notice so you can get time off from work. You could also change the locks so she would have to let you know the date/time of her impending arrival.