You can post a request here less then 3 days to your arrival OR if you got dropped by your host.
Be specific about the dates and times of your arrival and departure - that helps the host to decide yes or no.
If you write a post and find a couch, reply to your own post to tell the group you’re set
Request a couch either with a polite and interesting post or contact group members personally since not everyone is reading the posts
>>Make sure you have an interesting and complete profile.
If you travel as a group (more than two), raise your chances by showing that you are ready to split up.
Make sure EVERYONE has a profile and send links to their profiles.
Also try nearby cities like Potsdam or Frankfurt Oder! They are much less crowded and public transportation to Berlin is good.