First, some things to ask for up front

Always ask clients whether they have an in-house style sheet and/or word list. If relevant, ask if there are specific topically related reference books the client has and might want to lend you (for instance, an atlas of Alaska, if you're working on a travel guide to Alaska mentioning lots of rivers, towns, mountains, etc.).

If there is no style sheet/word list, make one as you go through the document, recording your decisions when they're not covered by Chicago/AP/Webster or when you fear you won't remember them as you proceed through the document and encounter the same issue again. Chicago has some examples of how to set up a word list, though I find it easiest to compile a running, alphabetized list on the computer as I go through the document; then it's easy to search, and it doesn't get messy.

Even if there is an in-house style sheet/word list, you will doubtless make your own additions to it as you edit.

Except with the shortest documents, try to get a copy of the text on disk, in whatever software you use, if you're working on hard copy. This makes it so much easier to search for something you think you remember seeing about 20 pages back.

Ask that the document be run through a spelling checker before it's given to you. Sometimes this isn't possible, but it doesn't hurt to ask. This can save you a lot of time, and thus the client a lot of money (a point to stress).

   Other tips

Ask clients whether they have particular concerns about the document—things they already know might be inconsistent, problematic, or wrong. This can also give you a sense of how deep an edit they're expecting (although it doesn't always give you a sense of what the document needs). Many clients think their documents are "almost perfect." But still, there's a big difference between "I think it's basically OK. It's been reviewed by two in-house editors and we just want to make sure it's perfect" and "I'm concerned about everything—grammar, spelling, sentence structure, organization, facts."

Ask whether clients prefer a particular dictionary. For instance, most publishers use Webster's Tenth New Collegiate Dictionary, though some use American Heritage. There are slight differences between them. If your client has no preference, then use whatever you like.

Ask whether clients use Chicago or AP Style (or another sort of style), and whether they use the serial comma. A lot of clients who use AP Style nevertheless also use the serial comma. If the client has no preference, you may want to ask about specifics: "Do you have a preference as far as whether numbers under 10 should be spelled out or in figures?" Once you see the document, it may be obvious which style would be best to use. Chicago is generally appropriate for books, other scholarly stuff, and stuff that wants to appear elegant; AP is generally used for a more general audience and breezy style.

Even if they want to use Chicago or AP (but especially if they have no preference, and especially with clients who are not publishers), don't get too hung up on enforcing one stylebook's rules exclusively. Keep in mind that most readers will not be familiar with any stylebooks, and clients may not want to receive a document back that's covered with red marks spelling out all the numbers when they were all figures to begin with—an issue most readers would not even notice or care about. On most matters, especially if your client is not a publisher, if the document is consistent or nearly so on some point, even if it violates a stylebook rule, consider going with the predominant style if it would not be offensive/confusing to the average reader. You can always query the client about specific consistent but unconventional styles that offend you ("Your document always capitalizes Federal and Government, but the accepted style is to lowercase it. Would that be OK?"). Sometimes clients are attached to particular idiosyncrasies, no matter what the stylebooks say, so it's a waste of effort to change them, and they won't appreciate your effort, and they won't like paying for it. It's probably best to save up queries like this and ask all at once, rather than calling each time about a particular point.

Make sure your level of edit is appropriate, especially if you're "just proofreading." For instance, if you're asked to proof a document that has to go to the printer in a few days, line editing is usually inappropriate. Some writers may be fussy and may not want their wording changed at all. If a book has been copyedited using an unconventional but consistent style on some point, don't change it to a more conventional style.

It's often helpful to both parties to give a client a rough estimate of how long you think the job will take. I usually copyedit 1,500 words an hour and proofread 10 typeset pages an hour, though of course this can vary widely depending on the quality of the writing and its density on the page. Giving an estimate is an especially helpful to clients who don't use editors a lot, as they may have no idea how long editing or proofreading takes. Their response may also give you a better idea of the level of edit/proof they're expecting, or that you can afford to give them based on their budget—a quick scan for spelling errors rather than an in-depth look at grammar, sentence structure, logical organization, etc., for instance. Estimate high so you can come in under your guess. Caution: If you're just starting out and have no idea how long jobs take, it's probably best not to try to give estimates; it could lead to more trouble than it's worth.

Especially when you're just starting out and are learning, you will probably have jobs where you don't feel comfortable billing all the hours you actually worked. This is regrettable but normal. Consider it part of your professional training costs. You can make it up later in your career, when you will be much faster, and will look back on these days and sigh.

Finally, it's always OK to ask the client more about what they want if you are uncertain.