In order to understand how to "preserve" PageRank, you first must understand what PR is. Unfortunitally, it is a complex forumula and includes among other things:
page content
Links in
Links out
page structure
and many other things.
The only way to fully keep the PR of an deleted domain, is to not let it expire and keep the content as it was. But as we all know, even that is not a sure thing!
Once a domain has expired and deleted and no longer displays a website, time starts to work against it. The site will be classified as "down" or "gone" depending on the search engine that tries to access it, and sites that link to it may start to notice that it's no longer active and may remove any links. Since the time required before you can register a deleted domain is about 60 days, there is a lot of time for the PR that was built up to be eroded.
Even when someone registers the domain, and even if they should put the same site back on the domain again, I think much advantage will have been lost. If a different site is placed on the domain, it may be better or worse than the old site, but I think the clock then starts ticking again and it will take sometime before the site realizes it's full potential.
I think there is some advantage to getting an deleted domain that has links to it, but unless the content is about the same, the links may continue to be lost.
I would be VERY interterested to see an deleted domain that was purchased that had any PR. I think some registrars put a parking page on some domains when they expire and keep that until it is renewed or deleted. But if you are talking about an "expired" domain which has been renewed and then sold to someone, that would be a different situation, but I think most of the things I have mentioned still apply.