The only sure way that I know of is to invest in an electric dehumidifier. Several people that I know have found that this solves the problem rapidly justifying the initial outlay. Lack of ventilation, inadequate air circulation and insufficient heating all contribute to the problem.
No doubt it's a north facing wall. This is the coldest of the house so any moisture can cause mould etc. You don't say you have mould or mildew but this can be prevented by keeping the room well-aired in summer and having a radiator on low in front of this wall. As Joke suggests there's also the choice of investing in a de-humidifier if the problem needs such an investment.