You want wild yeast cells to be able to get to your mixture, but you don't want anything else getting in it. So I think it makes sense to cover the top of the bowl with a dry non-lint dish towel.
I do have to say that I don't really like the idea of using salt and sugar to get a sourdough going. The salt inhibits yeast growth, and the sugar makes yeast go too fast. If you use organic whole wheat or whole rye, there should be naturally occurring wild yeast attached to the flour.
I got a very lively sourdough going a few months ago using a recipe from Peter Reinhart's Whole Grain Breads. He has two techniques in that book. One involves pineapple juice mixed with flour and water (I could be wrong, but I think that was it). The other one which I used involved creating a mash, which is effective but rather difficult.
There is one other way you can cheat. There was a guy named Carl Griffith who used to send sourdough starter to anyone who sent him a self-addressed stamped envelope. His culture supposedly went back to the 19th century. After he died, his friends picked up the torch and continue to send his culture if you send them a SASE.
Click here for info on how to get Carl's 1847 sourdough.
Keep in mind that every region has different strains of wild yeast, and no matter where you get a starter from, your indigenous wild yeast will soon take over, so whether you start with this or any other culture, it will eventually be populated by your own area's yeast, so it really doesn't make sense to pay a lot of money for San Francisco sourdough culture or any other. The key point is that it makes it very easy to get a culture going. Just mix with equal parts flour and water.
Edit to add: one other thing: most methods I have seen call for new additions of flour and water every 24 to 48 hours. Most also have you discard half of the previous amount when you make the addition, so that you don't end up with a ridiculous amount of starter. For example, after two days, discard half the mixture and add flour and water to get back to the amount you had previously. The reason for this is that you want to give the wild yeast some fresh food, and encourage those organisms that need to feed on fresh grains in order to survive.
