How to ensure scalability of logo design?
How can I make my logo suitable for all purposes, from webpages to very large printed banners?
I currently use use Photoshop.
e100's advice is spot on. A vector application such as Illustrator is the best tool for logo design, whether you start on paper and scan it in, or work directly within the program.
This isn't the whole story, however. Scalability doesn't just involve vectors; the detail in a logo must also be adjusted for the size of the finished artwork. Just as with text, fine detail will either get lost or appear too fussy at small sizes, and a logo that is drawn to look good at business card size will tend to appear stolid or clunky at very large sizes.
This is a bit tricky to demo on a low-resolution device like a monitor, but I'll try to convey the idea.
Here's a logo for a riverside community non-profit:
The wavy lines read well at this size, but look what happens when we bring it down to a business card:
You see the problem immediately: those fine lines are all but invisible. In print, you run a very strong chance that ink spread will swallow them completely. The solution is to make a version for small sizes with less detail, but that conveys the same impression:
In this case, increasing the text weight to bold is also necessary, because it is a subtly crafted serif face (Trajan) with fine detail that disappears at tiny sizes.
There is an excellent example of this in John McWade's "Before and After Graphics for Business." There is an entire section of the book devoted to logo design, and this is one of the books I recommend frequently for this and its other content.