I see what shops and fees are available in the region around me. When I see enough shops to justify a trip, I take them. Sometimes it's tricky when I have to request shops, knowing I may not get them but if I wait I may miss out on the others. Planning further out helps. If I end getting only some of the shops there is still time to find replacements.
I try to set it up so that I will get around $1.75 a mile if I have to go out of town. Around town, I shoot for $75 a day. Since I am on disability I cannot make over $800 a month until I reach 70 years old. So I only work 8 to 10 days a month.