A couple of years ago, when I was around 15, my Uncle asked me to help him do some stuff to a house he had just bought. So, I showed up the day we had agreed to meet, and I started doing usual chores like cleaning up a bit, moving stuff and the like. 

Even though at the time I did some things shirtless, I wasn't fully confident being so in front of people I didn't feel too familiar with. For me, it wasn't like when I went running (which I prefer to do during the evenings because it's cooler and there are less people out and about), or when I was at home, either in my room or around my brothers, in these situations I felt okay. 

Anyway, it later came time to work in the backyard and garden. My Uncle suggested I got rid of my shirt and handed me down a bottle of sunscreen, saying that it would be more comfortable considering the weather. I felt doubtful at first, but he insisted, and so, reluctantly, I took my shirt off.

I started doing the work and it was all going fine until my Uncle announced he would go out for a bit, later returning with a family – a man and a woman, I'd say between the ages of 40-45, and their two kids who looked around 11- or 12-ish. Suddenly I felt a bit uneasy. I said Hi and all, not forgetting I was wearing only just my running shorts, no shirt, also considering that flip-flops weren't really 'shoes', and that my body was all sweaty and maybe shining a bit because of the sunscreen. 

My Uncle introduced them and said that they were interested in buying the house; apparently he was fixing up the house with the intention of selling it and wanted to give them a tour of the house, but he needed to do some other stuff elsewhere and would be gone for at least 45 minutes – so he instructed me to show the family around and be a host. That wasn't the first time I had been in the house, my Uncle had shown my family and I around like a week or so before, so I was somewhat acquainted with the place.

I said "sure", adding that I would just go clean myself up and that we could start in a couple of minutes. However, my Uncle said No; that we should just get it over with, and the family agreed, saying they too were in a bit of a hurry. So, he took off and I was left there with them, and over the next half hour or so I showed the family around the house, including the bedrooms, the bathrooms, the living room, the kitchen, etc. you get the idea... all the while wearing only just my running shorts and my flip-flops, shirtless and covered in sunscreen, sweat and dirt.

After that day, I lost a lot of fears about being shirtless in front of strangers.

Source: Experience Project (site defunct)