Well, okay, maybe it isn't an AGE old question but it is a very oft asked one.
"Does writing short stories increase my chances to have my book published?"
I can only say, it doesn't hurt.
I have found that writing short stories keeps me in the writing chair, and that is always a good thing. Some people say that you must just sit down and let the words come (free-thought writing) to get into a writing mind-set. Writing short stories will enable that. Once you are working on what you think is going to be a short story, sometimes it turns into a longer work.
You never know when your characters from a book will decide they have a neat little story that needs telling, but you have no place to put it in your current WIP.
I will give them the stage and POOF! A new short story is born.
Short stories are not always meant to be sold to a short story market either. Sometimes they are just character sketches that you need to know about. Sometimes they are scenes from books you will write later. My suggestion is to keep a folder on your PC/Mac that will be labeled "short stories" and dump everything that isn't a book in the making into it.
If you like short mystery fiction, don't forget to check out the linkies on our page. There are a few good markets out there, and if you want to know how short stories are written, you can find a ton of good ones to read to help you in your journey.
I will have a new one out this month at Mouthfull of Bullets. GO -- READ-- and let me know what you think.