I'd second the advice to hold out for a Z Visa and Residence Permit. What you really want is the Permit; I'm honestly not sure if you can now start with that from outside China or not. But it's always been risky enough to upgrade an L (turista) visa to a Residence Permit; in today's climate it could be a recipe for disaster. Settle for nothing less than a Z before coming over!
Note that a lot of schools LOVE to have you come over on an L visa...typically obtained at your expense. It costs schools money to get you a Residence Permit...not a lot, but something. So, if you come with an L, they can make sure that you show up (admittedly a serious risk) and that they like you and that they didn't already hire too many teachers(admittedly a serious risk) before they invest the money in your documents. As long as you only have the L, it's easier for them to turn you away empty-handed at their door.
THIS REALLY DOES HAPPEN!!! More often than ya might think...
So...how to do all this?
First: recognize that you can't possibly 100% be sure of avoiding a bad teaching experience. You can stack the odds a bit more in your favor, but nothing is foolproof.
THE ODDS OF YOU HAVING SOME KIND OF DISPUTE OR PROBLEM WITH A TEACHING JOB IN CHINA ARE QUITE HIGH INDEED. There's always a risk in taking a job, especially in a foreign country...and very especially in a country whose culture is extremely different from your own country's.
This doesn't mean that you can't work it out (that happens sometimes) or that you will be physically or legally abused (that happens sometimes, too) or that you will be robbed by your employer (that happens rather a lot, unfortunately). It just means that the chances of sooner or later meeting some kind of unpleasantness are pretty good.
Then:
1) Spend a lot of time browsing around this website. There are vast stores of good info somewhere under all the cow pies here.
This process should include getting at least a basic knowledge of China's regions- geography, climate, relative wealth, and so on. If you don't see what you need, ask us!
2) Decide which cities/regions would be acceptable places for you to live. If you want, you can even create further rankings within your list. Again, there's help available here from both our posts, and the people who wrote them.
3) Decide what level of salary is the absolute minimum for you; can't hurt to be a little liberal in this judgment if you don't know much about life here yet. General rule- don't take anything under 4000 RMB plus a paid apartment in public schools/universities or in smaller cities and poorer areas; don't take anything under 5500 RMB plus a paid apartment in private schools or richer areas.
4) Scan the job listings for jobs that match your criteria...or at least come very close. Read them carefully...eliminate any jobs that come from a recruiter/agency rather than the schools themselves. You don't want to go there. Apply to the ones you have left. New jobs come up all the time, so you should repeat this step often.
5) When you get a response, research the school. Ask around. Search online for teacher reviews. Best of all, ask the responding schools to give you some e-mail addresses of some FORMER (not current!) teachers...current employees may have either positive or negative incentives to not tell you the full truth. If a school is unable or unwilling to provide these, taking the job means a risk that NO teachers have ever left them on good terms...
But ANY teacher review should be taken with a grain of salt. For one thing, some bad reviews are written by maniacs (and there are many of those here...) out for revenge, and some good reviews can be written by hired guns out to boost teacher recruiting and/or cover over a bad history. Also, conditions in a school can change in the blink of an eye, especially when there's been a change of ownership or management. However, when there are multiple reviews a decided drift in one direction or another can be a decent indicator.
There aren't always multiple reviews. There aren't always even one. There are ZILLIONS of schools in China; many of them have never been reviewed publicly. Just get what info you can...
6) If the research looks OK, ask to see the exact contract terms...preferably the contract itself. If you don't like some terms in the deal, try negotiating...sometimes it works. Note, though, that many public schools and universities have little or no legal leverage to negotiate salaries that are fixed by provincial Education Bureaus; negotiation here will have to be in on-duty-hour load or other parts of the compensation package.
7) If everything looks good, contact the school and tell them you accept the position. Ask them to send you 2 copies of a signed/stamped contract (sign one copy and send it back to the school) and the documents needed to get you either a Z Visa or a Residence Permit.
8: Start packin'. You're off to China!
God help you.
A final note...if it any time you feel pushed, deceived, or otherwise uncomfortable at any stage in the process above, you have the right to immediately cease all contact should you so choose, up to the point where both sides have signed the contract. (The school must also put its official seal on the contract!)
DON'T BE IN TOO BIG A RUSH TO COME HERE. Get a job with a reasonable chance of success FIRST...THEN come to China.
I may put a copy of this up in the Library...which is a great place to start looking for more information!