Laura Spencer, on Freelance Folder, posted a good article about negotiating with clients. She pointed out that many freelancers have a difficult time with client negotiations, and some tend to avoid them all together. I would certainly encourage freelance writers to negotiate with clients about price, time frame, and scope (amount of work) in order to be comfortable with your projects. Rather than immediately rejecting certain jobs or settling for less, you may be able to work out a deal that is beneficial to you.
Negotiations should always occur at the beginning of the project. The client may request a certain deadline and offer a certain amount of payment, but it is up to you to determine if you agree to those terms. If you think the deadline is tight, let the client know that in order to do a quality job, you may need more time. If the client is pressed to get the work done, try to negotiate a higher fee. Sometimes, if the client has a tight deadline and a limited budget, I will ask them if they would accept a shorter word count per article or something to that effect.
I wouldn’t advise you to negotiate when responding to an ad, however. You don’t know if they are actually interested in hiring you yet. That may seem a little presumptuous and can be off-putting. It is best to wait and see if they are willing to hire you, and then you can enter negotiations.
Don’t accept any project until you are satisfied with the terms. It can give the impression that you are disorganized or unprofessional if you agree to do a project, and then try to change the terms later. Many clients are willing to negotiate, and some actually expect that from you. Don’t be afraid to speak up, and be willing to compromise at least sometimes. That is not to say you should settle for less than you are worth, but if the client is willing to meet you halfway, then you should do the same.