So I noticed recently that while a lot of people here do some realy great art, no-one seems to be writing stories with it. Creating comics is one of the best ways to improve because it forces you to look at all areas of art, it keeps you motivated since you have a task with a foreseeable endgoal, and it poses lots of creative challenges. Don't just think of it as a means to be gooder at artisting though.
Don't think you have to nail regular drawing before you can start doing comics. If you think that, you will never do anything. You don't have to be good, you just have to be creative. Try not to just do moeblobs standing still and talking about how their mangaka is such a qt, make your images dynamic and try to communicate something like a story or a feeling.
There's a great series of blogposts called '18 tips for comic book artists'. http://www.williamstout.com/news/journal/?p=3806
(All those images are there for a reason, study them!)