I play this role in my team, and as a team lead I take it as one of my responsibilities. What works for us is that when I don't approve something I explain why

I play this role in my team, and as a team lead I take it as one of my responsibilities. What works for us is that when I don't approve something I explain why and work out an alternative implementation plan with them. If we cannot come up with an alternative or cannot convince ourselves that it is indeed better, then the original implementation goes in. Otherwise we move forward with the new plan. I'd say that that (when I challenge an implementation) around 4 out of 5 times we end up with a new implementation. The team is very happy with this approach, or so I've been told. It wastes some time in the short term (hey the thing worked, why are you overhauling it?) but our manager's perception of good team motivation and resulting quality is what buys me the leeway to keep doing it.