Remembering a dream for a long time isn't necessary the goal. Being able to wake up and write down MULTIPLE dreams is what you're really aiming for. But in order to get to that you must be able to remember good details with one dream. Eventually you will work your way up to 2 or 3 dreams a night. With the ability to recall more dreams, the more chances you have to becoming lucid and realize when you are dreaming.

Stick with it. It takes time and effort.