A new client of ours recently asked "Is there a way to store the values from two picklist items in the same field?". The answer, of course, is yes.
The problem is exampled below. Let us suppose we want to capture more than one Product Interest. By default, when you select a picklist item, it will replace the fields contents entirely.
But, if you put a ";" at the end of every picklist item, like this...
...then choosing a picklist item adds it to the end of the current field value, delimited by a comma.
Pro Tip : Everything after (and including) the "//" is ignored.