Caveat: I am writing this advice from the perspective of an editor who regularly works with freelance science writers. However, the market in which I work may not be the same as some of you out there work in, or want to work in. Therefore, I can guarantee that following the advice below will endear you to K-12 educational publishers (and to companies like mine who work for educational publishers). I can't speak to journals, newspapers, etc...but I can't imagine they'd mind if you follow this advice! And given that a lot of people here have expressed an interest in freelance writing, I thought I might be able to provide a bit of a different perspective on things.
Another note: Please do not interpret anything in this post as snarky. The points below are things that almost all of the freelancers I've ever worked with have violated at one point or another. Some of them are things that we see violated all the time. And I can say from personal experience that violating some of them (the ones marked with a star below) will almost surely get you kicked off any list of freelancers that a company regularly uses.
If you want to be hired as a freelance science writer (or, more importantly, if you want to be hired again as a freelance science writer :), here are some things you should always do:
*1. Keep your email address and other contact information up to date. If you have a website or blog, and that's how you advertise, make sure that people who go there looking for you can find you! If you've worked with a company in the past, send an occasional email to let them know you're still alive. (And it doesn't hurt to remind them what project(s) you worked with them on, especially if it was awhile ago.) If they can't get in touch with you, they won't offer you work.
2. Keep your resume up to date, and attach it any time you contact someone looking for work. Yes, most companies have a freelancer database, but it's not always convenient for the person receiving your email to search it to find out what you're good at. And even if they do have a database, it's always good for them to have an updated resume. (Oh yeah...and make sure there are no typos in your resume or email...)
*3. Turn your work in on time. Seriously. Those deadlines the company gives you? They're not just for laughs. If you don't turn it in on time, don't be surprised if the company doesn't accept it, doesn't pay you for it, or doesn't hire you again. Of course, life happens. Sometimes there's nothing you can do but miss a deadline. But if that happens, tell the company immediately, and offer a solution (e.g., "I can't get it to you on Friday, but I can definitely have it to you first thing Monday morning").
3a. Corollary: Unless the project manager specifically says otherwise, "due Friday May 1" means by 5 pm on Friday May 1, in the local time zone of the company. It doesn't mean midnight on Friday. If in doubt, clarify.
*4. Make sure your writing is good. I know, another no-brainer. But I have actually received files from people in the past that literally contained incomplete sentences. Even if you know for a fact that your writing is going through an editor, that doesn't mean you don't have to write well. If it's poorly written, the editor just has to rewrite it, and chances are they won't hire you again--because why would they pay you to write something that they then have to re-write?
*4a. Corollary 1: Make sure what you write is true and accurate. Do a fact-check before you submit. Even if you're writing for a 3rd-grade audience, you still have to give them accurate science. (I.e., it's not okay to say that a spider is an insect in order to make things "easier.")
*4b. Corollary 2: Even (especially?) if you are writing for a young audience, you have to write well. A guideline we sometimes give is "write for a 3rd grader, not like a 3rd grader."
*5. Follow any guidelines or directions given. Again...they do apply to you. If you're not sure about something, just ask! I've never met a project manager who would rather get something that doesn't follow the guidelines than answer a question or three.
6. Respond to all emails as soon as you get them. Even if it's just a short email from the project manager sending you a file to work on. Send an immediate response saying, "Thanks, I got it!" Otherwise, we're not sure whether you received it or it disappeared into the ether. And so we have to send another email asking if you got the first one...
7. Ask for feedback, and be gracious when you receive it. Maybe not in the middle of the project when things are crazy, but at the end of a project, email the project manager and ask whether your work was satisfactory, whether there was anything that you could've done to make them more likely to hire you again in the future, etc.
*7a. Corollary: Apply any feedback you get. If they tell you your writing was good but just a little too high-level, then the next time, make sure it's a little bit lower level. Applying the feedback the editors spent time giving you shows that you really are interested in giving them good work.
Here are some things you should never do if you can possibly help it:
*1. Quit a project the day before it is due (or, worse, the day it is due). If you think you're not going to be able to do a project you agreed to do, tell the project manager immediately. If you have to quit two days after signing on, you might make them a little grumpy because they have to find someone else. But if you quit the day something is due, and don't turn it in, you have just made their life a living hell, because now they have to write in one day what you were given a week to write. The former might get you off the freelancer list for a project or two. The latter will get you off of it permanently, or nearly permanently.
2. Demand more money. Obviously, sometimes the pay rate really is just too low for what is being asked. But if that is the case, please, present it nicely! I can guarantee that you will get a more positive response with a polite "I'm really sorry, but I expected this to take 3 hours and it's taken 10, so..." than with a "I can't do this for less than $X." It's perfectly fine to have a minimum rate, and to be clear about that rate. Just be polite about it. (And be realistic. As much as you might like to make $200/page, you're going to have a very hard time finding someone willing to pay that much. But if you do find someone willing to pay that much, PLEASE, let me know!!! :)
*3. Ask the project manager to bend the rules for you. Chances are, all of the guidelines the PM has sent to you were given to him or her from On High. The project manager can rarely change any of them. If you're told something has to be 300 words, it has to be 300 words. Don't turn in something that is 500 words and say, "It's impossible to make this 300 words." That doesn't help the project manager, because he or she still has to get it down to 300 words.
*4. Get grumpy with the client if he or she rejects your work because you didn't follow the guidelines or didn't turn it in on time. Let's face it: if you screwed up, it's your fault, not the PM's. This doesn't mean you can't (politely) argue something that you think isn't right (e.g., the guidelines say "3-6 pages" and you turned in 3 pages and they say you didn't write a long enough piece). But if they call you on something that you did incorrectly, either offer to fix it or apologize and let it go.
I know some of these seem like no-brainers, and I hope I haven't offended anyone. If I did, I apologize in advance :) But as I said, I have had freelance writers violate every single one of the points above at one point or another...so maybe they're not as no-brainer as we'd like to think.