In the US, you can use the Voice Training feature in the Alexa app. The option can be found by going to the Alexa app, selecting Settings, then Voice Training, then Start Session.
You will then be instructed to speak several phrases to help train your device to recognise your voice. Amazon provide the following instructions:
During a Voice Training session, say each of the 25 different phrases in the app to your Alexa device. Your device processes every phrase you say, even if you don't finish a session. For best results:
- Speak normally to your Alexa device.
- Sit or stand where you normally speak to your Alexa device.
- If you have a voice remote, do not use it during Voice Training.
The option seems to be unavailable in the UK, however (and presumably Germany also), according to this reddit discussion. In that case, you'll just have to speak as clearly as possible, and hope for the best. You might have some luck switching to the US language skills - if anyone wishes to test this, it would be helpful.
Also, be aware this this doesn't apply for devices using the Alexa Voice Service - only the Echo, Echo Dot and Tap are supported according to the documentation.