How to Be a Kick-Ass Female Customer at an Auto Repair Shop
Knowledge Is Power
Women who gather information about the repair's market rate, tell the repair shop they've done their research, and ask for a discount will likely get one. -- The Atlantic
Most people have access to the internet. Use it. Look up the repair you need so you know exactly what is going to happen and what parts will be required. This way, you'll know if they're trying to lump in unnecessary parts and repairs that have nothing to do with what you need. Even if you go into the shop not knowing what’s wrong with your vehicle, you can leave (without getting service) and do some research at home before committing to the repair.
It also doesn't hurt to become acquainted with the parts that are replaced/rotated the most often on your vehicle. That way, you'll know when to say no to "suggest repairs." I suggest knowing how old your wiper blades, air filter, and tires are--and also being aware of the last time your tires were rotated and your oil was changed.
Case in point: If a mechanic asks to replace your air filter (which looks fine) after only half of the recommended time between replacements has passed, you'll know to say "no."