The only way to communicate your problem and to be assured that the problems you wanted addressed are attended to is to have a written estimate. This written estimate is called a service order. It usually comes in several copies. One copy is given after you have discussed the problem and the writer has described the services and includes an estimate of the costs involved. The rest of the copies will stay at the shop to explain in detail what was done to your vehicle. After completion of the services or repairs a final copy is given that should be the same one you signed when you left the vehicle. This copy will have parts described in detail and the labor explained with the hourly time spent and the hourly rate you paid.
The estimate copy is the first thing to pay attention to. Make sure that it is filled out completely. Give work phone numbers and pager numbers so that communication can continue throughout the repair process. Make sure the descriptions you have discussed with the writer are on the estimate with symptoms and comments. Make sure that the price discussed verbally matches the written price on the estimate. There will be a box to check if you want your old parts(the defective parts), check this box before you get your copy. If you have any questions about services performed it is good to have the old parts available to aid in your understanding of what you just purchased.
The final copy of the work order is your receipt and a description in detail of the work performed. You should look for detailed descriptions of labor. "30K service" is not enough detail. You should look for what was and was not checked, adjusted, or replaced. Parts should be listed separately and prices given for each. Verify that the proper oil you prefered was used.(where applicable) Verify any symptoms that you described are now gone and that the repairs listed in labor are the same as what were estimated. Take a look at your old parts. You may not know what you are looking at but you can at least verify the defective objects and ask questions. The shop does not know how much you know, so at least you are showing an interest in the work performed. You never know in some larger organizations, if the person your dealing with even knows as much as you.
The service order is your only salvation in the rare occassion that you are not satisfied with the job and the facility refuses to make you a happy camper. If proper attention was paid to detail you should have all the information you need to make a case with an arbitrator or small claims court. In the case of a second opinion a well written service order can give the next technician helpful information. You as the customer have control over how well your service order is taken. Demand detail and give detail.
Leaving you car in someone elses hands is a big responsibility on both your part and the repair facility. Most service orders include an exclusion for theft and vandalizm that may happen while it is in for repairs. The service order is a legal document that is binding to both parties. My suggestion is to not leave anything in your vehicle that you value more than your relationship with your mechanic. Do a quick visual inspection of your paint, wheels, and interior to be sure you receive your car in the same condition that it was previous to repairs. Bring any problems you notice to the immediate attention of the person in charge. Be a part of the repair process by being involved with the repair facility. Give your facility the most important diagnostic tool. Your input.