Always Talk to Stangers, by David Wygant.
This is how I do it.
The book is about how to meet people, how to find dates, etc.
I recommend parents buy this book for their future missionaries when they are about 17 years old (or a mature 16.)
You don't have to tell your future (hopeful) missionary that this book is about missionary work. That's merely a stealth aspect of the book. But the techniques are basically the same, and they boil down to one thing: ALWAYS TALK TO STRANGERS. This book teaches you how to do it.
If you're a parent of a 17 year old, or a ward mission leader, I challenge you to read this book, and leave a comment.