What is the best way to say no to more work when you are already overworked and underpaid?

[Btaliat]

One things about the freelancing is that, freelancers should know then to say no. There are many offers that clients will bring. You don't need to take them if they don't have any value to you. For instance when you have many works with you, you don't need to accept any offers again. Doing this will make it difficult for you as a freelancer to cope with completion of the tasks. What do you think about this?