Aboalarm.de is very useful for cancelling contracts. You just enter the name of the company you want to cancel, then fill in the details, contract number etc, and then you can send it direct from the website (they fax it for you). It costs a couple of euros, and you could do it for free yourself, but it saves you a trip to the post office, and you have the peace of mind that they send it to the correct person etc. It won't help get you out of the contract any earlier though.
That probably depends on how much rent you are paying. An investor would have to take over your lease contract, but if you are paying a good rent, it might be attractive rather than off-putting for a certain type of buyer.