Test!  Test!  Test!  Don't get complacent and mail only to the same tried & true lists over and over.  Every donor acquisition campaign should include a number of new lists to test.  The number depends upon the size of your mailing and the availability of new files that match your donor profile.
 

Consider exchanging names with similar organizations rather than renting your entire mail quantity.  Not only will this reduce your donor acquisition cost but you will have access to the most recent and responsive names available.
  

Include recent segments of your lapsed donors in your acquisition mailing.  You will not only activate a portion of these donors but you will do it at a lower cost - you're mailing your own names!
 

Is the cost of doing acquisition mailings getting too expensive?  Then consider putting a portion of your lapsed donors on the rental market and putting this list rental income back into new name acquisition.
 

Consider remailing multis!  Multis are names that appear on more than one list.  These duplicates, also called multi-buyers or multi-hits, may be considered more valuable prospects than single record names, since each appearance of the name may indicate another giving opportunity.  In addition, you've already paid for these names, why not mail them!