Avital's Sourdough Starter for the Super Lazy:
Mix 1 cup of water with 1 cup of flour. Leave the bowl out overnight with a handkerchief covering it to keep ants out. If it bubbles, you've got a starter.
I add another cup of water and cup of flour, stir, then put in the fridge in a container with a loose lid. It will need feeding every couple weeks or so.
I should add that I find it easier to catch yeast in the Judean desert than in Boston, so YMMV. Generally it's easier in summer than winter, too. In summer the mixture will start bubbling like a milkshake within a few hours. In winter sometimes it takes a little more than a day.
If you have a vintage starter with a pedigree, it is a good idea to keep some frozen and occasionally throw out your current starter and replace with the 'new' starter from the freezer. The reason is that the local fungi (including yeasts) will eventually crowd out the foreign fungi. That is why starters vary from region to region.