I am doing that very thing right now; in fact I have 2 courses in progress and then 1 more course to take in the spring semester, and my degree will be done! One tip I will give you is that it's been a lot of work! It's nice to be able to log in and work whenever and where ever I have time, but the online classes are every bit as demanding as in-person classes, and in some cases more so.
You need to be a self-starter, and you need to set aside time every day to work on your class assignments. Online classes do not lend themselves well to procrastinators.
You need to be organized. I create a folder in my laptop or each class, and then in those i create sub-folders for assignments pending, assignments complete, reference materials, etc. I also copy/paste all my discussion posts to a word document and save them, which allows me to go back and refer to them in subsequent classes.
My courses are through a traditional brick-and-mortar state university (in fact, it's the same place where I got my Bachelor's, through a standard 4 year residency program). You need to research your choices and choose a school with a good reputation; in my case, my diploma will have the name of a respected university, and no one will be able to tell whether I did the class work online or in person. In some places this is important, as some administrators have a negative attitude toward the online-only degrees, thinking they are somehow lesser quality.