TSteven hit it square on the head: Traditions. Someone told me that constant, repetitive contact is the key, but I think that's pointless unless the alumni can tie it into something they're familiar with.
One thing that can really help: have alumni be responsible for the alumni news. Undergrads don't know anyone beyond their own age. Alumni who know the history of the chapter can write the stories, and that's what keeps the memories fresh. Here's a good example: see
www.fsupikes.com and click on the Alumni News.