We cannot overemphasize the importance of planning your release schedule. The biggest single thing that hampers a label's success - after the quality of the music itself - is not giving yourself enough time to properly set up your releases.
Why spend a year negotiating, tweaking and perfecting your label's big release, only to fire it out a couple of days or weeks after it's been delivered?
Always have the next three months' worth of releases planned out, giving yourself, artists, PRs and anyone else involved enough space to really get behind the releases. That's not to say that if someone drops a hot record in your lap you can't move things around to accommodate it, but setting a schedule - however loose - will pay dividends.