As Peter's and keshlam's answers point out, an up-front contract goes a great length to help here. I agree, except that for tasks that may take max 4 hours, and perhaps are needed in a rush, this might be a bit of an overkill.
Even if it's just to find out what the problem is, my auto mechanic makes me sign a rough generic estimate of what it's going to cost every time I bring my car to him. This is not just for his benefit, it's for mine as well since it outlines the maximum amount I'm willing to pay (unless I specifically authorize a higher amount later on).
And if you can't ask your client to sign such a document while in a rush, or return you an electronically signed copy of such a document by email. It probably just means that you don't have one already prepared and ready to go at a moment's notice. Ideally, you should always carry blank copies of such an agreement in your office, in your bag, in your car, and in your phone over email.
I have done such small tasks for others in the past without much more than a verbal agreement. In the end I am more worried about reputation (customer sastifaction) than the money or time, as the former is largely a mental construct, that therefore scales more easily to larger projects (where the money may be better).
Money and time are not just mental constructs. If you don't take care of getting your client to sign relatively simple contracts for a couple of hundred bucks here and there, it will probably be that much harder for you to do the same when contracts involve actual real amounts of money and real commitments of several weeks.
That's how you can scale. First master the right behavior for the small amounts. Trust me, those small amounts are actually much easier to get a client to commit to. And once you've mastered those small contracts, then that same behavior will come in handy once you go after the bigger contracts.
If you want to be a successful professional, you really do need get over this hangup. You can't pay for rent/mortgage/food with pats on the back alone. Also when first getting started, don't be above asking for small amounts of money from your clients.
Also, the quicker you can figure out that a client isn't willing to sign a small contract, the quicker you've figured out that the same client would never have been willing to pay you for a larger contract either. Some people are just born that way. It's not their fault. You don't need to change their mind. You just need to find out who they are so you can avoidweed them like the plagueout as quickly as possible.