Make sure the backlinks to the expired domain are in the hundreds. Most webmasters will not be alterted to the fact that the website is no longer. The links will remain for a long time.
Try to get expired domains with listings in dmoz.org which takes its sweet time removing sites.
Try to get domains that are relevant to the new site. Not necessary but works the best this way.
PR should retain.
edited to add:
a) engage in a massive link exchange with relevant sites – pronto – build the momentum.
b) incorporate old websites keywords so you don’t loose SE listings the old site may have had.
c) make sure you launch immediately after obtaining the domain, don’t wait around for a week to make a site