1. Location, location, location
The type of tattoo you design will be limited, in large part, by the area of your body it's going to be applied to. If, for example, you're designing a full back piece, you'll have a lot more lee-way in adding design elements than if you're planning a small tattoo for your ankle. In addition, if you work in a professional field, you may want to consider a somewhat tame design if your tattoo will be placed in a location that isn't covered up by your clothing. It's sad, but some business people may prematurely judge you if they see a skull and crossbones sticking out over your collar.
2. Think balance
One of the biggest aspects of good tattoo design is balance. You want your tattoo design to have a certain visual flow to it - lopsided or unbalanced tattoos tend to look unprofessional. So consider adding circular elements to your tattoo design, or work with mirror images to give your tattoo a more professional look. If you're having trouble implementing this concept in your design, ask a friend with a graphics arts background to give you some pointers. Remember, spend some time on your design - you'll be wearing it for the rest of your life.
3. To color or not to color?
Deciding whether or not to add color to you tattoo design is no small matter. All black tattoos tend to age better over the years, since some colored inks tend to degrade at different rates. At the very least, you'll need to plan for periodic touchups throughout your life to keep the design looking its best. On the other hand, all black tattoos can look harsh against the skin - especially if you're very pale - so it may be in your best interest to consider incorporating color into your tattoo design.