Creating an index to include ORDER BY

[Angus]

I have a query which at it's most complex looks like this:

SELECT * FROM t1 WHERE DateColumn<NOW() AND NumberColumn>=const1 AND NumberColumn<=const2 ORDER BY DateColumn,ThePrimaryKeyColumn

Sometimes either or both of the NumberColumn expressions in the WHERE are missing, but the DateColumn and ORDER BY are always there. What index do you recommend? Is there an index that I can create which would include the ORDER BY?