Milestone payments
I do what I refer to as milestone payments on big stuff.
Lets say a group of large trees in a back yard competely removed.
Milestone A would be after limbing and topping out,
Milestone B would be after chipping all debris and hauling off chips to the compost,
Milestone C would be dropping the trunks and hauling off,
Last but not least, D would be stump grinding and final site cleanup.
Each milestoine in this example would be a 25% payment.
I personally do not ask for any money up front. I will always post a milestone before requesting any payemnt. I just think it is good policy. Also, many customers are leary of money up front operations and you may lose the job due to consumer paranoia.