One thing many translators (and other people who run their own small businesses) tend to worry about is what will happen if they turn down a job. They fret, saying things such as, "If I say no to this particular job, the client will never ask me to do anyting again. And s/he won't recommend me to anyone else either." This leads to a situation where many translators (myself included) take on more work than they handle and thus find themselves stressed and overworked. Okay, I can admit that I personally prefer being stressed and overworked to not having any jobs at all, but it's actually not an ideal situation.
So is it true that if you say no to a client once, that means never hearing from him or her again? I would say that this depends on how you say no. Do not say no without explaining why. And always say you are looking forward to hearing from that client again at some point.
If you are turning a job down because you simply do not have the time, explain that, and make sure you add, "Thank you for asking me. I hope you will think of me again in the future."
If you are turning a job down because it is not in your area of expertise, recommend an appropriate colleague (or give a link to where the client can find a translator, such as the Swedish Association of Professional Translators) and remind the client what your particular speciality is.
The point is that even as you say no, do so politely and helpfully, while also subtly telling the customer you will be available in the future for other assignments.
It's true that once clients have found a good translator who delivers on time, charges a fair price, and is professional to work with, they might be unwilling to switch to someone else, partially due to inertia. So if you say no once and the client finds another translator who fulfills the requirements, you may not hear from that person again. But if you are selective about which projects you take on, which clients you choose to work with, and when and how you turn down assignments, it is likely that you will build up a stable of customers who return to you over and over again, even if sometimes you have to reject certain jobs.