easiest/most general thing to do is to use different browsers (so that her cookies [the little bits of information saved on the computer, so that it can track who is logged in), so that her cookies are saved in her browser, and yours in yours.

Next easiest - If you both want to use the same browser (such as Firefox or Chrome) that supports multiple user profiles, then set one up for each of you.

Finally, if you're forced to use a browser that doesn't support multiple users (such as Internet Explorer), then you just need to tell the browser to clear all cookies (each time you switch between her account and yours).