commercial payment method

I recently started my own power washing business. I've only had a few jobs but they were commercial and payed very well. My question is do I ask for a downpayment on big jobs? Ive done one for 10k and another for 15k and luckely they paid. But then I did a golf corse parking lot and these guys took forever to pay. It made me think about finding an answer to this problem before I do a big job and dont get paid for my services. So, how do you guys do it.

Thanks

what kind of power washer do you have?