Because you aren't throwing away your diapers, you can't use just any diaper cream. Many of the cloth diaper safe diaper cream. There are many brands available and they work great!
mainstream diaper creams sold at your local drug store contain ingredients that can be problematic for cloth diapers. You want to avoid creams that contain any fish oils or petroleum based ingredients. When using many of these diaper creams, you may find build-up and repelling with your diapers.
If you find yourself in a pinch and must use a cream that isn't cloth diaper safe, a liner is suggested. It is also important to check the diaper manufacturer's recommendations when choosing a cream. Some brands say that no diaper creams are safe (even cloth diaper "safe" creams) and you should always use a liner to protect your warranty.