This excellent article from Unbounce draws upon the data from a selection of academic studies on copywriting plus persuasion to provide five best practices for improving your writing skills.
Here's a quick summary of the key points:
1. Use the 'But You are Free to' Technique
Reminding people of their freedom to choose has been shown to reduce how threatened subjects feel about making that choice.
2. Copywriting for the Three Types of Customer
Customers separate into three core buying groups — tightwads, unconflicted, and spendthrifts. Therefore, you should customize your copy to appeal to these different types of customer.
3. Making Use of Mirror Neurons
This essentially means to focus your copywriting on how the reader is feeling and thus target how they imagine something to be — for example, how they feel about a problem that your service helps to overcome.
4. Use the Power of Verbs
Use verbs rather than adjectives in your copy as they are harder to ignore — i.e., say what your product does rather than how you would describe it. For example, say that your software application searches terabytes of data in seconds rather than just that it is powerful.
5. Write Your Stories Like a Lawyer
Apparently, there are six characteristics of persuasive stories that perform best in the courtroom: delivery, imagery, realism, structure, context, and audience. These can be applied equally well to writing persuasive copy online.