Mapping Center Answer:

You can use a SQL query to restrict which features are labeled.  From the online help:

"Using label classes to label features from the same layer differently

1. Open the Label Manager.
2. Click the layer in the Label Classes box for which you want to create label classes.
3. Type a name for your new label class in the Enter label name box and click Add.
4. Uncheck the Default label class to avoid labeling some features twice.
5. Right-click the new label class in the Label Classes list and click SQL Query.
6. Click the operators to build an expression that identifies the subset of features you want to label.
7. Click OK.
8. Click the Label Field drop-down arrow and click the attribute field you want to use as a label.
9. Click the buttons and drop-down menus to define the symbol and placement properties of your labels.
10. Repeat steps 2 through 9 if you want to create additional label classes.
11. Click OK."