I want to buy a ton of ladybugs and let them loose - just for the fun of seeing that many swirling around me :) .
Anyway, my answer...Flowers! My son planted sunflowers and it has lured tons of bees, butterflies and ladybugs. Plus at the north end of my garden I have a flower garden with different types of flowers - just to look pretty, they weren't planted with any other reason in mind. I put marigolds and borage (which REALLY attracts the bees like crazy) and other flowering herbs mixed among the beds as well.

Rachel~Our plum tree was really infested with aphids (I think that's what they were) so I bought 2k ladybugs for $10. The ladybugs cleaned that tree right up! I still see a few floating around the yard, and I've seen some of the babies (that don't look ANYTHING like the adults) traveling clear to the other side of the yard searching for bugs to eat--I put them on my veggies. I don't know how many have stuck around, but I don't really see a lot now.

Oh, and my peppers had consistent visits from aphids until I surrounded them with marigolds. Now the aphids are hanging out on my lettuce--away from the marigolds. So there's proof that the marigolds work to keep the peskies away. ;-)