-Use a color scheme. If you use too many colors, typically your designs will begin to look tacky and unprofessional. This does somewhat depend on the business type, however. If you're creating a logo for a toy store, this would be an exception. But you'd definitely want to stick to only a couple colors if you're designing for an insurance firm, for instance.
-Keep it simple. Try creating logos that convey as much information as possible while also being as simple as possible. This is the mark of a professional designer. Having a simple logo will help in a number of ways, including the ease of distribution. For instance, they are easy to place on letterhead and business cards because they don't require much space to be recognized and understood. If your logo is the Mona Lisa with some text slapped on top of it, you would always need the logo to be quite large just for people to be able to see what it is, and this is the kind of thing you will want to avoid.
-Make it memorable. You want viewers to become used to a logo, and they will do just that if you make it unforgettable. Think outside of the box and try to create a unique quality that's never been seen before. Making your logos memorable will, if you are or wish to become a designer, help you in the same way it helps the business it represents, with recognition. If your logo is well-known, that's a great mark for the portfolio.
-Use a font that compliments the style of what it represents. You aren't going to want to use an old-english type font for a pet store logo, and you aren't going to want to use a child's handwriting for a debt consolidation firm. Just use common sense in this area. You can also search the web for free fonts if you aren't quite happy with what you've got. There are a bunch of websites out there that contain lots of great community-designed fonts.
-Make your logo fit with different background colors. If you've got black text and a black icon, they won't show up if you have to place them on a black background, so create multiple versions of your logos to accommodate for different background colors. If it's imperative to stick to a certain color, then you might try adding a stroke (border) to your design to make it show up better, or perhaps placing the logo on top of a solid-colored rectangle of inverse color. You always will want your logos to be clearly visible so make every effort to keep them that way.
-Use vectors. Create your logos using vectors (paths) as opposed to pixels if possible, so you will be able to scale them up at a later date with no loss in quality. This way, you will not run into any issues if you ever need to print out your logo that you've created as a 200*100 pixel file onto a poster or banner. Insufficient size when you're stuck with pixels just isn't very easy to recover from. The only options you would have would be to either ignore the problem and scale it up despite the bad quality, or completely redesign the logo at a higher resolution, and you're simply not going to want to do either of these.
If you stick to these guidelines your logos will start looking very attractive, but don't take them all at face value; rules are made to be broken!
Barrett Phillips is webmaster of Baphi.com and Author of “Simply Design - Aesthetics That Sell". His book offers easy tutorials on Graphic Design that will get you producing professional-grade graphics in no time? http://www.baphi.com/