For example, if we want to find donors who donated in 2007 and 2008, but gave less in 2008 than in 2007:

1. Create a constituent, dynamic query with the criteria of:
Summary:  Total Number of Gifts_1 greater than or equals to 1
- Gift Date equals This Calendar yr
AND
Summary: Total Number of Gifts_2 greater than or equal to 1
- Gift Date equals Last Calendar yr
2. Save and close the query
3. Open a new Comparative Report, comparing this year's to last year's giving and marking to create an output query
4. On the Format tab under Detail for Variance to Include, mark Decrease. (Or Increase to see those who gave more) 
5. Preview the report to receive the prompt to save the query.