Design Patterns for Data Access Layer

I have an application which uses a database (MongoDB) to store information. In the past I have used a class full of static methods to save and retrieve data but I have since realised this is not very object-oriented-ish or future proof.

Even though it is very unlikely I will change database I would rather something that does not tie me too strongly to Mongo. I would also like to be able to cache results with the option to refresh the cached object from the database but this is not essential and can be done in another place.

I have looked at data access objects but they do not seem very well-defined and I can't find any good examples of implementation (In Java or a similar language). I also have many one off cases such as finding usernames for tab completion which seem not well suited and would make the DAO large and bloated.

Are there any design patterns that would facilitate getting and saving objects without being too database specific? Good examples of implementation would be helpful (preferably in Java).