This is probably old hat to most but I thought worth sharing to the uninitiated as I previously was.
After buying my M240 late last year I was plagued with a constant dirty sensor. Cleaning became a weekly necessity. The camera seemed to be a magnet to dust spots and grunge. With no local cleaning facility I had no alternative but to self clean. Every proprietary brand proved useless. Sure, after much effort I could achieve a "sort of" clean sensor but never really satisfactory and soon the issue returned. I got to the point of selling the camera.
Research led me to Isopropanol. Leica also recommend this product. The problem was that the 100% proof was/is unavailable over here, BUT, they have the 70% available in most pharmacies which is used to clean cuts and burns. A small bottle costs just $2.00. I was nervous, but went for it. Two small drops on a Visible Dust pad, a quick wipe one way and back on the dry side and bingo, a shiny, clean, pristine sensor. Relief is an understatement but the best was yet to come as I established that this fluid contains an anti dust/grunge element to prevent reoccurrence.
It's now been 3 months since my last wet clean and having shot the proverbial blue sky at f22 this morning I see absolutely no trace of anything on the sensor despite having just returned from a very humid dusty shoot in the Thai outback with constant lens changes.
Hope this may help others with the problem.