Multi selection list. how to multi select

Greetings.

I just came to this need.   I stored values in a field as a comma separated list, which represent selections of a multiselected list.

How can I show throug a list on a window, which  (or all of them) where selected before (I got this in the db stored list in a char field).

What I mean is:   I have a list of selected values, for instance: "one,two,four,six", and want to show a list, wich in turn has this options: "one,two,three,four,five,six,seven", but want to be preselected the former values... (for now the list is single values).

Thanks id advance.