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I don't think there's any any of making that show all the time. It's just when google last looked at your page and put it in it's page cache, if it was during the last couple of days they'll show a date, otherwise they don't - some people add a hidden field with todays date to try and track it...
From what I see the sites with higher PR have the page cache refreshed more frequently (however it may be that they are really dynamic sites - updating a lot etc, I haven't really checked), I've got a couple of sites, that usually have the home page cache updated at least every other day, and inner pages updated a couple of times a month (I'm sure there's a few people here that do better than that :)) - hope that kind of helps....