I wrap my left hand around the spokes either side of the rim. That way I am pumping into my left hand and the whole wheel, not just the valve. I make it a point to ensure the pump goes on cleanly and at right angles to the valve. (Zephal HPX pumps or Specialized floor pump.) I also make sure the pump gasket is completely past the valve tip. Then I see to it the Zephal pump stays that right angle to the wheel as I pump. I am not perfect and small bends happen, but small bends are rarely an issue.

For thirty years all my tires were pumped with Zephal frame pumps. For the first 20 those were all sewups (and got pumped frequently). I don't think I ever lost a sew-up to a broken valve. Try my hand position. And like others, I am a big fan of pumps with thumblocks and won't own a pump without (although mostly because the gaskets last so much longer).

Ben