How much do you maintain your ranked MNS sites?

I know the answer is relative. But i have a few sites, ranked 1-4 on page 1.

One of them has been siting there earning for 4 months and I've done nothing to it since then. Yesterday i've noticed it dancing(maybe wrong term) a bit between position 2-4. What should i do to reinforce it? new article, more links, same things to rank it?

I'm about to go into building in bulk, no longer 1 or 2 a month.

I was wonder what you do to keep your rankings. If i have 60 sites, how much maintenance am I looking at? These are low comp keywords, typical MNS stuff.

Basically, I can rank MNS now, but i'm not sure what the long term plan should be. Maybe keep each one for 3 months, then sell it, repeat?

Thanks,

~Darren

Just for a note, if it is a relatively new site i.e 3 to 6 mths old, when you suddenly add backlinks to it, it might happen to dance abit or even few pages. This always happened to me when my mns rank in page one with little or no backlinks. Do it slow n steady. A better approach which i tested quite useful is to add facebook like, add tweeter followers, add google plus and also buy some traffic concurrently with your backlinking campaign.