There is no sure-fire way to make builders respect you.
Being, or looking, young gets you off to a bad start. The site manager on a job of any size will generally be a grizzled graduate of the university of life who talks about his last experience of dealing with an inexperienced architect as if he had been through the siege of Leningrad. Not going on site, instead dealing with his scrawled fax enquiries in beautifully typed replies full of limpid prose may seem like the only option here to regain the moral high ground, but it has the drawback that in the final account meeting you won't have a clue what has happened during construction.
Oddly, being old doesn't seem to help much either.
However matily you can talk to the foreman about pre-metric plumbing fittings, he'll still run rings around you by claiming to have found blue asbestos in places that the most agile 10year-old chimney-sweep would struggle to inspect, and citing 'new European regulations' as an excuse for not building what's on the drawings. The experienced architect's trick of arriving on site early to disturb the programme of routine deception will help you prevent the worst excesses.
However, if the average site agent is to believed, which, admittedly, he's not in any other respect, only about 05.30 would count as early, and then you might well be asleep by the time the other consultants arrive.
Outright hostility from site managers is only slightly worse in the long run than over-familiarity: your favourite ketchup on the table with your bacon sandwich when you turn up on site will, rightly, make you suspicious but, more insidiously, a professed fondness for the same football team, or a copy of the AJ prominently but casually displayed should, equally, alert you to his wiles; either he's trying to get your agreement to some particularly bold variation or he's stalking you.