The phrase "Improve a Marriage" suggests the marriage is lacking something. I think the most important lesson I've learned from being married 25 years is that each person in the relationship has to communicate their needs to their partner. Honest communication with each other means your partner doesn't have to guess what you want or expect to get from him/her. Sit down together, open a bottle of wine and have an open and honest discussion.
As we get older, our needs change, our desires change and that can put a strain on a marriage. This is especially true when you feel like you don't even know them anymore. A slumping marriage may be a result of losing touch with each other so it is important that you don't leave it to chance that your partner will figure out what you want. All you need to do is to make sure they know and then keep that line of communication open forever.