Many of these firms know who their paymasters are. They had been able to develop very excellent working relationships with their clients that make them kept returning with more assignments. Even though they have excellent working relationships, they always clarify the scope of works required right from the beginning of the project.
This is very important because many clients have high expectations for their consulting engineers. It is okay, to be honest with them for what you can or can not do. If a particular project or task beyond your ability, you have to admit your own shortcomings and advise them to hire other experts.
You may end up losing projects to other engineering consulting firms, but still better than to secure a job but can not do. Many customers appreciate your honesty and openness and will likely come back to you for other projects. You also avoid the risk that entangled in a lawsuit filed by a client.
When you first submit a proposal to potential clients, You should always be affirmative in describing to clients about why he should engage you instead of the other engineer. Present your obvious benefits compared to the costs involved.
Some consulting engineers also committed the mistakes of spending too much time trying to convince a prospective client. However, there are no fixed rules on how much effort you should spend. Just stick to your business sense and let your intuition guide you.