You keep doing it as often as you can until it stops making you uncomfortable. Photography is hundreds of years old and most people do not give a shit about having their photos taken. You care more than they do.
Everyone feels the nerves when they start street photography and since most people are pussies who give up at the first hint of adversity, they don't continue on. The trick is to putting yourself in that discomfort until it doesn't bother you anymore.
Also, don't be a sneak. If your body language is full of anxiety or you sneak photos of people, it makes them more suspicious than if you just take a photo then smile politely after. People respond to threatening body language more than anything else. If you're sperging around or acting nervous, people will think you're up to something and react defensively.