Here are 3 ways to prevent your website from losing ranking in major search engines:
1) Keep adding fresh content – this includes new pages and inbound links.
2) Keep analysing your competition. See what they are doing right (and wrong).
3) Always have a long term strategy in mind. For example, you’ve ranked successfully for keywords A, B and C. You need to keep the ranking for those but think about where your business is going and how to get there.