How do I NOT clear memcache or other cache tables? All of us who love Drupal know the pain of building those cache tables for the first time.

I have a site whose activity is high during a certain part of the day and then non existant at others. I have over 25,000 items of content on the site, and that grows daily.  I  have Memcache, APC (etc) installed for performance. When the site is active, it flies. I am very happy with the speed and performance.

However, if you are part of the lucky few who get to access the site when the cache tables are being rebuilt, you have a good 10 min to sit there and wait.

What can I do to prevent this? Can I have memcache keep all cached files in memory until new versions of pages are requested? I thought about writing a spider to access the site every 15 min to half hour or so (somewhat like boost module does), but wanted to see if any of my caching options could be simply configured before diving into that. If so what configuration options would you recommend?
One setting you can play around with is the minimum cache lifetime setting. It will help, but it won't solve the issue.

The standard Drupal cache doesn't allow for what you are describing. One can do some exotic stuff to update caches in the background but unless you have a large developer budget to make this work I would move on. Note: The 6.x version of boost is the only thing I know of that allows for the cache to be updated in the background.

If you are looking to build a spider, http://drupal.org/project/httprl/ is the way to go. I hope to be developing with this more in the future.