How I did it: This was pretty exhausting. I ended up keeping a fair bit - I shipped four boxes home in the end. I sold a lot of things on "kleinanzeigen.ebay.de" (which is like kijiji) at really low prices.
I made a bit of money and it was great German practice. I think I spoke and wrote more German in the last few weeks while selling all my stuff than in the rest of the two years I'd been living in Berlin.
A few days before we had to have everything out, I was starting to get worried about how I'd get rid of all the little things like cooking stuff and utensils. So I put the rest of my furniture up for free. A couple came an hour later to take our free chairs and then we offered them whatever else they wanted. Comically, they pulled out enormous fold-out zip-up bags and filled them with as much as they could carry. Then they came back the next day and took the rest.
On the last day I rode home from work through the park and gave that guy my bicycle, we slept on a blow-up mattress that we bought on a beach holiday - the kind for using in pools - and put our freshly cleaned sheets and quilts in a clothing bin, and that was it.
It was such a relief to finish cleaning up our place.
Lessons & tips: Make sure to set very specific times for when people will come to pick up your stuff. Otherwise you end up waiting around the house too much.