If you like ambitious, disruptive approaches then you could donate to the SENS Research Foundation in support of the research strategy known as whole-body interdiction of lengthening of telomeres (WILT) [1]. People who follow cancer research lament that all cancers are different, and that there are few biochemical commonalities. This is what makes cancer expensive: even with the next generation of fairly general targeted cell killing therapies that disconnect (a) the kill mechanism from (b) the delivery mechanism and (c) the targeting mechanism you have to build and prove new targeting mechanisms for every subtype of cancer. Slow going, and this is by no means fully realized as a technology base at this point.
But all cancers do in fact have a commonality, just one you have to be ambitious to target: they must lengthen their telomeres, either through abuse of telomerase or the less well understood alternative lengthening of telomeres process. If you can shut off these few mechanisms for a sufficiently long period of time then any cancer dies. Doing this temporarily seems plausible. Doing it permanently would mean you never get cancer, but you'd also need all of your stem cell populations replaced every decade at least, or you'd suffer some form of accelerated-aging like condition due to absence tissue maintenance, which is a whole other situation of complexity that lies somewhere in the future. One by one it's pretty plausible to replace stem cell populations, and indeed technically possible today for some of the better understood stem cell populations such as those of the immune system, but there are a lot of them to do. A lot.
So if you want to help the development of the ultimate cure for cancer - some form of interdiction of telomere lengthening - then send a large donation to the SENS Research Foundation and earmark it for that line of research. You can look at their annual reports to see where they are in this research.
http://www.businessinsider.com/small-amount-of-money-from-pi...
http://www.huffingtonpost.com/nancy-stordahl/breast-cancer-p...
Does anyone know the best places to donate?