Make honeymoon last keeps on fire burning

First you both need to decide to keep the honeymoon alive. The main reason that people do not keep the honeymoon feeling is that they do not decide to. You have to consciously commit, early in the relationship, to spend the time, energy and effort to keep that honeymoon feeling. This should be the type of commitment that all of us feel good about.
Second, just like the actual honeymoon, kiss her -- a lot! Even if you've been together for years and years, you should kiss her good morning, kiss her goodnight, and kiss her when you get home from work, or when she gets home from work! Your minimum should be at least three times a day. If you're not displaying your love for her in a physical way and keeping that constant reminding going, you will have a hard time keeping the honeymoon feeling alive.
Just like kissing, tell her that you love her. This also should be done at least three times every day! It is very important not just to know that you love her, but also to verbalize your feelings on an ongoing basis. You did it during the honeymoon; continue to do it now and throughout your relationship.