I am a management consultant, so field of work might be different from yours, but general rules are probably the same. Also, I am not a freelancer, though i have worked often with them. My two pence:

- Always have a Scope of Work (SOW) agreed BEFORE you start working: Clients are often bad at explaining what they want in detail, and consultants are equally bad at being realistic at what they can offer in the amount of time available.

- Always (ideally) have a contract signed before you do any substantial work. The contract and the SoW are your main protection against scope creep. And usually it is the consultant who drafts them first, not the client, so it is your best chance of driving the project details.

- Rates vary with client, length of the contract, stage of client relationship (lower rates for first project), but ...

- Rates tend to be sticky. If you work for Xusd/h for the first project, many clients will expect that rate to stay for the next one too. So your first negotiation is the more difficult and the more important at the same time.

- Rather than offer a low rate, state your full rate and the discount you apply and why (e.g. first project, long project, maybe the client is putting some resources and tools on the project...)

- Preferably charge by hour, day or week. The proposal or contract should have an estimate of total price, based on estimate of duration, but it should clearly state that it is an estimate and you will change based on time units.

- However, it is very likely that yours is not the only offer they receive, so they will compare them on the same basis, based on a fixed amount of hours per day, and the duration of the project. A consulting firm staffing a team has more leeway with the composition of the team (junior consultant to Partner), and comparisons among firms are slightly more difficult (usually done on the basis of blended rate). The quotes of freelancers are very easy to convert and compare.

- Expenses are always on top (travel, hotels; meals are a question mark)

- Try to find out hourly or day rates of consulting firm (in you field of activity) in your country: they are likely a lot higher than what you think of charging, partly because they have higher costs, but also because they factor in a % of idle time of their resources during the year. You should do the same. Your rate will probably still be lower then theirs as there is a real value in being part of a firm with established methodologies, expertise, examples... (this may be less applicable in other fields of consulting )

- Think about why they need a consultant: is it because their resources are too busy or because they do not know how to resolve the problem they have? The latter gives you more leverage on the rate.

- Remember that every client is different: some impose work based on a fixed total price, some do not want to reimburse expenses (and may be fine with a % markup on the rate). Often it is due to internal procurement rules and guidelines, which will be difficult to change.

- Bill often throughout a project (frequency depends on project duration: weekly for short assignments, monthly for longer).

- A good practice is to send a fee tracker weekly to the client, showing hours worked and incurred fee. This avoids surprises. It is best if you have some deliverables or progress report to show as well.

- If you feel that the project is taking longer than expected, be open about it and explain why (it may be because some input from the client was delayed, and they usually do not object to additional compensation, or at least they will push to accelerate on their side; it may be because things are more difficult than expected: this may become a difficult conversation, so be prepared to explain why it is so, what can be done, etcetera).

- Your project is also the best occasion to get another project with the same client (as a follow-on activity or maybe because while working on it you get to know they have other problems that you can help with. It is BY FAR the best form of business development.