Did you use a subscribe page to get them to sign up? If so you could add the attributes to the page, then get them to fill out their preferences using the preferences placeholders (they are described in the manual and in the info pop-up box just above your editor. You may need to offer some kind of motivating reason, like a prize or free stuff.
Don’t forget, even if all of your subscribers don’t have attributes, you can still use the attribute placeholders, you can do like