Logos: a few things to think about
The Design
HOW your image is designed is important!! The best and most professional way of reproducing a logo in any way you might need it is to use Vector art, and save the file as an EPS. Vector Art is different from a scanned image, in that it's not dependant on staying the same size in output as it was scanned in. Scanned images CAN be resized, but lose quality the bigger or smaller you try to change from the original size.
Vector Art is usually created in Illustrator, or in the newer versions of photoshop that have the capabilities. When saved as an EPS, you can then open the design for a large poster or a small web graphic.
Number of Colors
This is a big issue when designing a logo, since with each color added, the price of reproducing it increases. A good rule of thumb is to remember to choose a design that looks good in all of the different ways you might need to use it now and in the future.
Paper Output:
For business cards, letterhead, posters, etc....
Usually you will want to head to a professional printer for these things.
Even if you choose to use 4 color (or full color) on your design, remember that a fax (as well as copied flyers, or handouts, etc.) are limited usually to ONLY solid black ink on white paper. Many fax machines don't have the capability to recognize greys, so your logo will look horrible coming through on those machines, and there is no way to do an easy "fix" on these if your logo is designed dependant on more then one solid color.
Cloth Output:
Things like T-shirts or fabric printing add another interesting element to the design of the logo, in that you can pay for more then one color, but each color you use must be a SOLID block (or line or shape) you can't feather, or lighten the colors in the image. Also, you need to be carefully on fine details here... each line weight (width) must be sufficent for the ink to get through the screen and onto the shirt or article. Again here you will pay additionally for each layer of color that is needed to make your logo.
Monitor Output:
Web Use, Powerpoint presentations, and other such computer viewed projects, is of course the most flexible, since you can use full color, animation, and just about anything your heart desires. This is usually the smallest concern, since monitor output can usually only enhance, and spice up your logo... also, you aren't charged by number of colors.
The one limitation of this type of output is that the web has a very limited number of colors that it can accurately display to all users. So when choosing your color(s) make sure that the ink version, and the web version of those colors match close enough for your needs.