Always maintain continuity SEO activity like-- directory submission, bookmarking, article posting, making blog and posting,classified submission etc..
It's easy to reach at top but difficult to be there for long time. Same for the site once site reaches at good position keep doing SEO for it regularly.to keep it as it is.
Keep updating new contents and do your off page optimization. Try to get good backlinks from high PR. Keep monitoring you keywords ranking in the SERP.
update your content, make link building continuously for your website from best quality backlink, submit your content to social bookmark or directory
If you want to maintain the position is necessary. So, you can do directory submission, blog submission, link building, forum posting and article submission and all the techniques of SEO.
You know dude, because of the updates happen most often, there is no specific tip in maintaining your rankings. The only thing that you can do is to continue what you're doing or remember the methods or techniques that brought your site to the top and focus on it.