Also, you can tie your query's parameter to an object on your Form.

For example, say there's a combobox on your form that the user can utilize to select a value, and this value is what will be fed to the query that is your subform's record source.

In your query (in Query Design view) you place this for the parameter's value:
[VBA][Forms]![FormName]![ObjectName][/VBA]
Where FormName is the form's real name and ObjectName is the combobox's name (or whatever object you are actually using). Then you need to click on Query-> Parameters- enter the exact same thing in the first box, and then select the data type that should be sent to the query.

HTH