How to fix .DOC and .XLS sharing problem

Have you ever noticed the annoying window that pops up when you try to open a microsoft office file that someone is already in? It asks you if you would like to open a read-only copy. Is there any way around this? The main reason why this pops upfor our company is when we look at our to-do lists. They are in Excel format and the bosses are always adding to them while we try to complete them. Therefore, the file-sharing/access problem occurs rather frequently. Does anyone know of either away around this issue or a good to-do program that avoids or provides the solution? Please advise as I would like to get this issue of my to-do list soon! Thank you.