Here are some friendly tips about general technique.

Tips

Tricks

Troubleshooting

I keep hitting my arm!

First thing to do is make sure you’re wearing an arm guard (also called a bracer). This should protect your arm against it.
However, that’s no fix. The main thing to prevent string slap is (bow) arm rotation.
In order to comfortably and easily rotate your bow arm, your bow hand must be placed in the proper position on the grip.