Keep roast beef fresh?
I'm going to assume you are talking about deli-style sliced roast beef here. There are a couple of ways to help preserve meat freshness better.


Wrap it in butcher paper. This type of food-grade wrapping paper is inexpensive and can be purchased at many general stores and office supply stores. It works great for all sorts of meat, poultry and even fish, and can be used in the refrigerator and freezer. 
Keep it in the coldest section of your refrigerator. It is often near the top or the back, varies depending on refrigerator model.