Adverbs, such as firstly, secondly, etc.

I often have to create documents that include a long list of reasons why a position in which I am in favor is the correct one. Reasons one through ten seem properly started with "Firstly," "Secondly," "Thirdly," etc. However, when I get to the teens, using the adverbial form of a number does not sound at all correct. When have you ever heard someone say, for example, "Seventeenthly, it is your fault because..."? Does anyone have a learned opinion about the correct way to attack this problem?

Thanks, Slowlearner