This post is designed as a step-by-step working for a requester you never heard of until working for them for years.
1. Most important thing is look for hits you can do and enjoy doing.
Even if the hit pays extremely well and the requester is a great requester, if it's not the type of hit you want to do it shouldn't be a consideration. Some people don't write well so they shouldn't attempt writing hits. Some don't dig transcription so they shouldn't. If you're at work or home with small children you probably don't want to do anything that might have adult content.
2. Research Requester.
Don't be shy about spending too much time researching a requester. A year down the road you won't be working for 50+ requesters, but more like about a dozen. So it's a long term investment researching requesters. It may take a few hours today and you might or might not find a new requester. And that's fine so long as you keep good books and remember who you've researched and why you said yes or no. It might also be worth to write on a notepad why you made a certain choice. You may find out that your initial opinion was unfounded or things changed.
The best place to research a requester is to use Turkopticon. At the time of this post they are changing things a bit and making things better, so hopefully it will be better. Currently I wouldn't pay much attention to the scores. I'll save the reasons for a different post. Read all the comments and make up your own decision. You may find a requester others don't like and vice versa. The "scare" words you should look out for is "block", "mass rejection", etc. These are terms that can affect your account so it typically isn't worth is.
Another place to research requesters is forums. Some people prefer forums since it's more of a discussion. Problem is if the moderator is ban happy they may ban users that don't agree with them in some other aspect. Another problem is a thread might be 100s of posts, most of them pretty useless. The benefit, though, is you can see who is posting what, and if you know who the poster is it might give you more of a reason to trust their opinion over someone else.
3. Doing a hit
a. Read the directions. Understand what it is the requester is wanting you to do, and start doing the hits.
b. After about a half dozen hits, reread the directions. This is simply just to make sure you are doing it right now that you can put the hit to the directions. Last thing you want is to do too many and realize you forgot an essential step. This happens.
c. After about a captcha's worth of hits - 35 hits - read the directions again. At this point you are going by the motions. Like step 2 you just want to make sure that there isn't something super minor you glanced over and aren't doing.
4. Communicating with a reqeuster
Maybe you found out that you missed some step. As soon as you know you should contact the requester and admit your error. When I do this I ask the requester to reject the hits I did wrong. I'd rather have more rejections rather than a block on my account. At this point you should stop working on the hits. You don't want to get even more rejections if the requester mass rejects you. If you do get rejects remember you were the one that did something wrong so don't be mad at the requesters.
Sometimes for a brand new requester I'll email them asking some question I might have just to see if they respond and how quickly. If they respond while the hit is still up it's probably safe to assume they are good to work for and won't mass reject you, though that's always a possibility.
5. Minimizing your losses
Many people have a similar story "I got mass rejected 200 hits, now my approval rating is 89% and I can't find any work". Maybe you didn't deserve the rejections, but it's partially your fault for doing this many hits for an unknown requester.
You should know the percentage required for all hits you normally do and you should never have more pending than what will drop you below that number.
Lets say you have 30 rejections currently and 1200 hits. Also let's say the highest requester is 95%.
a. Simply divide your approval rating by your highest requester. 1200/.95. This will give you a number of total hits that will give you 95% if you don't get an accepted. 1200/.95 = 1263.
b. Subtract a from your approved and rejected. 1263 - (1200 + 30) = 33. At max you should only do 33 hits from an unknown requester. You may not even want to do that many. So alter the goals by .99, .98, .97, .96. For this example you won't consider .95 because you shouldn't risk your ratings that much and prevent you from doing work for your normal requesters.
Once these hits have been approved you can begin to wok on the negative side - that is, 94%, 93% etc. Depending on how safe you want to be should tell you what to reject. Next set you may want to shave off a percentage or 2, or if you want to risk more you can do 90%. You shouldn't assume that since the first 33 was paid you can now do 1,000. You should gradually build up to these numbers.
If the requester is well known with great feedback, you should still limit the first batch just to make sure you are doing it correctly. Also, just because you don't get rejections doesn't mean you are doing it correctly. That's why you need to build up towards numbers.
6. Even after working for months for this requester, you still need to watch your numbers. Requester may decide to mass reject you to avoid paying you. It happens. They also may decide to mass reject and block you because you aren't meeting some computerized quota or something. Again, this happens. You'd think after working for a requester for a year that this won't happen. There are stories about requesters purposely rejecting workers after having a long relationship. This may be due to hitting the $600 mark. It may just be they want fresh eyes and just don't want you working for them.