I have noticed alot of the old links I have slowly disappearing as the pages they sit on in many Link directories are being deindexed by google. Its hard to expect pages that have little action on them stay in the google index. Once they disappear out of the index they rendor the link juice to my websies useless. I have often thought having a sitemap of inbound links will help google remember where they are and keep them alive longer but wonder if anyone has done it or had another strategy to keep inbound links alive?
try to keep in mind that google pretty much makes it policy to NOT show you all of your backlinks. google publically announced that with the change to the new webmastertools interface that it would be showing a smaller portion of your links to you. Thier reasoning was something like "to make it more user friendly". But regardless of that horse-shit you need to remember this one thing. Don't obsess about the links google shows you. Obsess about results. Where are you in the serps, how much traffic are you getting. If those things haven't gone downhill then don't sweat the small stuff, just keep doing what you are doing. building links and adding quality content.
Old links do disappear, especially links from forums and directories and sometimes blogs. For me Google Webmasters shows a decrease in my backlinks soon after a toolbar export. Then in the week or two up to a new toolbar export of PR, my backlinks jumps up and some that had disappeared return again.