From Laura Ries:
1. It is better to be first than it is to be better.
2. A #2 brand that uses a better claim is rarely a big success.
3. If you can’t be first, set up a new category you can be first in.
4. PR builds brands. Advertising maintains them.
5. The target is not necessarily the same as the market.
6. Names matter.
More detail here.
