Just imagine getting a phone call or an email from someone asking you to go work on a translation project for them. You will probably think, yes, of course, I would love to do that, right?
However, before making a decision, try to get as many information about the project as possible.
First of all, ask to see the source material before committing to anything. You never know what you will end up with.
Second, make sure that the source material lies in your field of expertise. Ask yourself if you really feel comfortable translating, for instance, legal or medical documents. If the answer is no, decline the job offer. In the end both you and your client are better off.
Third, once you have decided that you want to go for the project, be honest to your client on how much time you really need translating. Never make any unrealistic promises on how much work you can take on.
Fourth, always deliver on time or even before schedule. This is absolutely the professional way to do it. Your client will be delighted and may come back to you for future projects.
All in all, if you consider those very simple steps, you can establish a mutually rewarding working relationship with your clients.