A recognisable pattern in small businesses that struggle with cash: the work is done first, in full, and the invoice goes out at the end. Materials are bought, days are spent, and the business carries the entire cost until the customer pays, which may be thirty days later or may be whenever the customer gets round to it. If payment is slow, the business is now chasing money for work already delivered - which is the weakest possible position, because the only leverage it had was the work itself, and that has already been handed over. Every option from that point is worse than the one it just gave up.
A deposit changes the position, and it does so in more than one way. It covers the materials, so the business is not lending money to its customer to buy things on their behalf. It establishes that the customer is serious, which quietly eliminates the enquiries that were never going to become jobs and which currently consume time in quotes and site visits. And it starts the relationship with a payment that went smoothly, which turns out to matter: a customer who has already paid something once is markedly more likely to pay the rest without difficulty, and a customer who finds a reason not to pay the deposit has told you something valuable while it is still cheap to learn.
The obstacle is almost never the customer. In most trades a deposit is entirely normal and expected, and customers who commission work regularly are used to it and often surprised when it is not asked for. The obstacle is how it feels to ask. It feels like distrust, like an admission that the business needs the money, like something that might lose the job. That feeling is real and it is worth naming, because it is what keeps the practice from being adopted, and no argument about cash flow addresses it. What does address it is realising the framing is backwards: the business asking for nothing up front is the one signalling something unusual, not the one asking.
Making it routine removes the moment of decision, which is where the discomfort lives. State it in the quote as a matter of course rather than raising it separately afterwards - a stated percentage on acceptance, the balance on completion, and for longer jobs a stage in between. When it appears in the quote as an ordinary term, the customer reads it as how this business works, which is what it is. When it is raised afterwards as a request, it reads as a reaction to something, and invites a conversation. Same money, same terms; the difference is entirely in whether it arrived as a policy or as a question, and that difference is free.