Come up with a good approach to the situation. In other words state what the cause is and where the money or items would go. Also make sure to have a good attitude when you do it. Happy and enthusiastic tend to work better then pushy and desperate :)
Give them all the information, as to what the "cause" does, how much of the donated money goes to the charity, and not administrative costs , then, they will decide, if its something they want to support!