I have a few rules I try to abide by when complaining:

I try to do it as soon as possible - for example, in a hotel I am not going to fume for the entire stay but instead try to get it resolved so I can enjoy the rest of the trip

I try to be reasonable in my expectations, and my expectations for resolution.

I try to work with the person helping - in some situations, a refund of money is a lot harder than, for example, upgrading a room, comping the parking, getting a bottle of champagne delivered so I am willing to see what e.g. the front desk agent has the power to do if it would be satisfactory to me. I thank the people who helped once the issue is resolved to my satisfaction. In some instances I have sought out their manager, to complement them on their staff member.

I try to remember that the person I am speaking to is rarely the one who caused the issue (I have seen too many people lay into front desk staff, waiters, FAs etc.) If they lack the power, seniority, or simply language abilities to help, I will ask for management but I find it more effective giving the first point of contact the respect that they are authorized to help.

I try to keep my tone moderate and my body language neutral.

I try to be factual about the complaint and avoid rhetoric.

When all else fails, I have no problem cracking out a more hard line version of myself but honestly I find I have to do that very infrequently.