Sticky, I am in a niche and most of my work comes word of mouth. the work I do is either already a repeat customer from previous pleasant experiences or will hopefully be one after the order so I generally just put in the time on their design and have it on file for the next order. If I was doing one-off work online or some such I would do it differently. The other thing to consider is after you get some experience under your belt most designs don't take that long. Some are a mother and I can spot them ahead of time and charge for it if I think I need to. Most of the time if they are ordering at least... say... $50 worth of product then I do the vector work for free. I still make sure they know I am throwing in something for them so they understand some else might charge for that otherwise I would lose the benefit of giving it away. If they have a design they want me to copy I make sure my version of their design is basically an exact match. That way there is no question about whether they like the look or not.
I have had occasion to have someone want 1 shirt or something like that. I do those on a case by case business. Some people are a PIA and nothing makes them content and you can spend literally hours back and forth with emails. Most of the time I try to be pleasant to work with and about half of these people are now regulars and have mellowed out on their picky ways. The other half I have upped my prices and some moved on and the ones who stayed I make enough to feel like it's worth my time putting up with a PIA.