Writing short stories is an excellent way of honing your skills. Several noted authors have recommended the form:
Ray Bradbury: Write a short story every week. It's not possible to write 52 bad short stories in a row.
Lorrie Moore: A short story is a love affair; a novel is a marriage.
Neil Gaiman: A short story is the ultimate close-up magic trick—a couple of thousand words to take you around the universe or break your heart.
I've been wading through the intricacies of formatting e-books for digital publishers recently, as well as reading up on how best to query and write synopses of my novels. I've been down in the dumps, feeling the lack of creative writing, but yesterday I had the happy thought that I could create a couple of short stories featuring my Cornish Detective, to give away to anyone who signs up to my newsletter for my still-to-be-created website.
One of the things I like about short stories is the 'fast to fail' characteristic. With a novel, it can takes you months or years before you realise you are on the wrong track. With a short story, the learning is so much faster, the lost investment so much less. So, although I like them for themselves, I also think they are excellent training tools. That said, I suspect novels require skills that short stories don't, and perhaps vice versa, but nevertheless there is useful overlap.