You can do it without any coding when the combo's RowSource is a single-column field from table/query, which is the source of form with this combo. I.e. you don't use lookup table, but select any earlier entred value from field of table.
An example:
You have a table SomeTable, with a field SomeField, and a form based on this table. Put a combo into form with field SomeField as source;
Set the RowSource of combo as query
Code:
SELECT DISTINCT SomeField FROM SomeTable ORDER BY SomeField ASC
Set the LimitToList property of combo to FALSE (NB! To be able to do so, the RowSource query must return a single field - at least this was the condition when I last tried this!);
Set the BoundColumn property of combo to 1, and the ColumnWidth to e.g. "2.5";
It's all!
To get same functionality for combo using lookup table needs some amount of VBA (I have seen some posts here how to do this, but I never had any need to do this myself!).