short term apartment in Berlin

I have no personal experience with this outfit but plan to try them later this year. http://www.ferienwohnungen-berlin.de/
If you click on the british map upper left corner, it should come up in English. Perhaps Jo in Frankfurt will respond; I think she has experience with Berlin apartments.

Hi Dennis, I just got back from a week in Berlin and I rented the Capri apartment from berlinhabitat.com. I was quite satisfied. They were easy to do business with, I loved the location, and the flat was as advertised. The two main reasons that I chose them were location and their favorable payment terms. I booked through their online site on a weekend using a credit card for the deposit. I got an immediate auto-generated confirmation and on the following Monday I got an email from Stefan the owner. The owner sent me instructions on getting to the apartment and since he was not going to be around at the time that I arrived, he arranged for me to pick up the keys at a bakery in the building next door. They did not require a damage deposit, and they allowed the remaining balance to be paid by credit card for an additional 10 EUR. This was very convenient for me as Berlin was the only Euro location that I was in, so I didn't have to "stock up" on Euro to pay the balance before arrival. I really liked the location. It was around the corner from the Rosenthaler Platz u-bahn station and an easy walk to Hackescher Markt and the Museum Insel. There were lots of small cafes in the area. The owner had info in the apartment with the nearest, ATM, grocery store, restaurant suggestions , where to put the trash, etc. I wouldn't hesitate to rent from them again.

Not sure if this apartment (cosyapartment.com) will fit your needs, but we just spent a week there and loved it. I believe they are listed on VRBO, too. The apartment is exactly as it appears on the website. The owners (who live in the apartment next door) could not be nicer or more helpful if they tried. The location (Tiergarten) is very convenient (you are literally in the Schloss Bellevue's backyard and 1 stop from the Hauptbahnhof) and at the same time, in a non-touristy area.

You can try berlinlofts.com Good price for lots of space in a renovated old stable building.

We spend a lot of time in Berlin. We've used some of the sites noted above with good luck. Also try Homeaway.com; we found a good apartment for 7 weeks in Berlin this summer with them.