The web has made it too easy to build a spreadsheet and email-bomb publications with your stories. But just because you can submit your stories to one hundred magazines in a single keystroke doesn't mean that you should do that.
For my money, the biggest mistake that freelance writers make is not reading the publications they submit to.
Before you ever submit someplace, you should already love the editors and writers published on those pages--you should know exactly where and why your submission would fit in that magazine before you ever push the send button.
But don't take my word for it. Isak's inspiring writing site has a link-filled discussion of the topic, with some great thoughts about people who submit to the literary journal, Virginia Quarterly Review. Don't be in the 99 percent of thoughtless writers.
Check it out: "Ross White (who first tipped me to this) lays the matter, finally, to bed: 'I don’t see what all the fuss is about since 99% of VQR submitters have never seen the magazine. Hell, 99% of all submitters anywhere have never read the magazine.'"