The unsung hero of GoldMine; the F2 lookup!
By which I mean the simple picklist that every field has. Geeks like me call them F2 lookups, because if you're in a field, hitting F2 brings up the picklist.
Did you know that the mere addition of a semi-colon to the end of a picklist item allows you to add it to the end of a fields' value?
For example, if I have a picklist with the following items;
...choosing any of these sets the field to a single value. If you add a semi-colon to the end of each choice, like this;
...choosing any of these will append the value after a comma. So your field will look like "Widget, Gizmo", etc.
Remember that if you use this method, you'll need to change your Filters to use the CONTAINS operator, instead of EQUALS. Have fun!