If you are offering advisory services only, you should propose a retainer structure, or a possible 50% retainer, 50% on milestones or completion. You could also do a monthly retainer based on the length of the project. If you are actually doing some type of implementation work, I would expect you could ask for a retainer/deposit and then bill the rest on time and materials.

How does your industry typically structure deals?
Will the customer focus on value-based billing or effort-based billing?
Are you managing a team of people?
What is your seniority and level of experience?

Without knowing more about the project, your experience, or the nature of the work, it's hard to answer this question but I would be happy to chat further, having many years of experience building and operating a professional services firm. There are many options around pricing models - the best fit depends on the project and the customer (and you!). Feel free to reach out to me directly if you would like to discuss further.

Do not continue to exchange hours for dollars. It is a losing game. You can never win doing this. Bill only by the project. And get paid according to percentage of completion. In addition, charge a royalty whenever possible.

Charge $A up front. $B per month, and $C per %completion, $D as an ongoing royalty, and $E for future contribution which varies according to your contribution.

I hope this helps.
Best of Luck,
Mike

PS I may assist you as my time permits. Right now I am doing something that I haven't done for many years. I am actively soliciting new select clients due to selling one of my businesses and freeing myself up a little. But as you might expect, I am still very busy. That is how I like it.
TheLittleGuysWayToRiches.com

This is a very easy answer. If you can't scope so that you have a very, very solid target on the work required to finish the project then you have to price it by time/materials.

There is no other sound way to do it and not lose your shorts. The bigger the project, the more the risk/variables, the more complex it is to scope.

I like the approach of "here are my best estimates of how far we'll be after 3/6/9 months, here is what it will cost, and here are the resources I have to commit to it. "

Cheers,
Ryan