I agree the Ripley idea is great. But general practice with unfamiliar situations and big scary dogs (dog school) would also be good. I know my dog thinks all Golden Retrievers and Kelpies are her best friends - until she gets within sniffing distance and realises she's never met these ones before. She's pretty good about it. But her reactions to old men who want a pat - seems random to me. Some she greets immediately like long lost friends and others she just doesn't trust. And the more eager they act the more suspicious she gets. She's usually best with the ones that have their own dogs - but not always. Sigh. She keeps just out of reach and if I'm really unlucky, sneaks up behind and sniffs butt (goosey). Sheesh.
I sometimes wonder why those ones are only interesting from behind.