at the bottom of the feed item there is a list of commands and one of them is "Keep Unread"

or you can just press the m
key to toggle the read/unread status of that item
In regard to items not being available to mark unread after 30 days (i did not know this)...one approach to get around this is to use a service such as instapaper to save articles for later reading.. and to go even further you can grab an RSS feed for your instapaper account and load that as a feed into Google Reader so you can see all of your "unread" items in instapaper in one feed.. this is what I do