Multi-selection value list

I'm looking for ideas for how to present to the user a value list from which he can make multiple selections. This is the "Categories" field of a Contact Manager - a given contact may belong to an arbitrary number of categories.

The value list is long (30-40 items) and it may be extended by the user, so whatever interface element I use should accomodate its size and variability. I think this pretty much rules out the usual checkbox interface.

When a Contact record is edited, assigning a category should by default add to, not replace, any previous categories. But it must be easy to delete a category from a contact record, too.

I almost got a workable solution by creating a Category field several lines tall, with a scroll-bar, and formatting it as a Drop-down List (+ Include Arrow, + Include Edit, + Autocomplete). The problem is that, by default, the interface tends to want to replace previous values: navigating into the Category field auto-selects all current values. To add a value, you have to first un-select, by hitting right-arrow for example. It's too easy to mess this up.

Can anyone suggest a method for managing a list of this sort?

Thanks,

Chap