Personally, I use Harnesses for the two terriers because (even though they dont pull a lot now) when they do pull it helps stop them from choking themselves. Plus its a lot harder to escape from a harness than a collar, and escaping from anywhere is Squids speciality

With Toby I use a regular collar and leash. Because he's bigger, if I put him in the harness and he decides to pull (only when we see a strange cat ) he puts his whole weight in the harness and I can hardly hold him.

When I was training them, I used to make them turn a circle every time they pulled. Squid spent the first 20 minutes spinning, so he soon decided it was easier for him to walk next to me