What is the best payment-structuring model to best align the interests of the person requesting the work, with the person doing the work, while also de-incentiv

What is the best payment-structuring model to best align the interests of the person requesting the work, with the person doing the work, while also de-incentivizing things like scope creep and simply "one requirement ends up taking too long and should be considered for dropping prior to completion"? Is it half upfront, half on delivery? Is it fee-per-milestone? Is it hourly rate? Is it some hybrid of the above? When a requirement must be dropped, who pays for the hours already sunk into it? Would it be both parties? (by charging half the normal rate)