1. Don't request it. Earn it.

You may remind the clients to give you rating but it is not wise to request them to give you favorable ratings. Evaluating your performance is clients' right and you should leave it to them. Even if you do a good job, requesting the client to give you a better rating may result in a negative effect.

2. Good translation is necessary but not enough.

In addition to delivering quality translation, you must demonstrate the professionalism when working with the clients. Be prompt, timely, and responsive. Follow the instructions, but give advices when you see something wrong. Be courteous and friendly.

3. Don't fool clients.

Always keep the same quality standard regardless whether clients know the languages you translate or not. Even if they don't understand the languages, they have many ways to verify your translations, e.g. searching on web or hiring another translator for QA. Further more, as your translation will be published or shown to some readers in the end, the clients can still change the rating once problems are exposed.