1. Make sure that your illustration is absolutely necessary. Can the objective be completed any other way?
2. Get examples of what the client likes. Ask the client many questions about what they want: Colors, techniques, objective. Ask the client what they don't want: Colors, techniques, representations.
3. Take the client through the process. Send the client your drafts. It will keep them in the loop and diagnose any problems early. This will also help you achieve an approved final product quicker.
4. Have confidence in your work. If you are proud of your work let the client know. Your positivity and confidence will help you sell your product to the client. If you question its quality, then the client will also question its quality. Confidence and praise will sell your work.
5. Keep all of your work for reference later. If another client comes to you with an illustration request, what is better than your previous work to establish expectation? The client will be getting used to seeing your work, and this will ultimately prepare them for the work you do for the best.