Clients are often surprised that first meetings between their aggressive dog and I go so well. I often hear, "Wow, normally he doesn't warm up to people."
Is this some magic power I have? No. But I have trained my body to always always present non-confrontational body language to dogs who don't know me. When I'm greeting them properly (or letting them greet me!), all things go smoother.
If you have a dog that seems reactive to new people, try coaching family members and friends to greet a dog politely. Here's how!