Get several written estimates for any job. Have a contract for the work to be done. Have the contract checked by a trusted relative or attorney before signing. Check with friends and the local Better Business Bureau before you enter into a contract with someone. Ask for written guarantee for the work.
Verbal guarantees are worthless.
Ask for old parts back on auto and appliance repair. Make sure the parts you pay for have actually been replaced. Don't exhibit large amounts of cash when shopping or banking. Carry only what you need to conduct your transactions. Don't wear expensive jewelry or watches if you don't need to while shopping.