Learn from mistakes
Theory has its place, but an example often makes learning something much easier. In many areas, an example of a mistake or poor quality is an even more effective teacher than examples of the correct technique.
Using examples to teach
For instance, I can tell you it is best to use the fewest words possible to give a message and to avoid repeating a word.
Or I can give an example: Leave as long as possible before proof reading your writing.
Or I can show you a bad example: Another effective way to increase the possibility of increasing your link building purposes… Then explain the issues with it and write it well: Another effective way of potentially increasing your incoming links…
Does it work for you?
Do you like seeing poor examples of something as a means of learning to avoid those same mistakes yourself?
I have put some bad writing examples in my blog (and the one above is a real example from a blog post I read) and always include one in my newsletter.
The bad examples I use are real but I never identify who wrote them – if you searched hard enough you might figure it out, but I respect that the writers didn’t mean to provide us with bad examples and use discretion 
I think it is an effective way of showing how to write well – but do you find it useful? Would you like to see more bad examples I spot to help you improve your writing?
Writing a complete list can be tedious, so we tend to write out part of a list as a sample instead. Implying it is a sample even when we think we have written out the entire list, can also be useful – it protects you from giving an absolute.