Just a few reminders of things I've learned over the years:
1. Don't pay to submit your stories. The exception is reputable contests with large prizes and reasonable entry fees. Don't pay "administrative fees." (Optional donations are another matter, but are a grey area - the donation button should not be linked to the submission button).
2. Never sign over any rights on submission. Even if they say you'll get them back when they reject you (what if they never respond?) Do not enter newspaper writing contests without checking the fine print - newspapers are absolutely notorious for claiming first rights on everything submitted. This goes for photography contests too.
3. Follow the guidelines. Mistakes happen and won't be held against you, but a repeated pattern will. As frustrating as "no simultaneous submissions" is, follow it.
4. Do not sign a contract that doesn't have a rights reversion clause if the company goes out of business (why yes, I did learn that one the hard way). Publishers? Please have such a clause. Otherwise your writers might be held to a lengthy exclusivity period on a story that isn't even available and for which they never got paid. Also avoid novel contracts that demand first refusal on anything you might write ever again even if it's not their genre.
5. Don't give your work away for others to profit from (unless it's for charity). Consider marketing freebies very carefully - and if asked to provide something for a convention package or other giveaway, ask if it can be a reprint (which costs you essentially nothing). Don't devalue your work.