Close your left eye. Bring your sights up, focus on what you normally focus on (typically focusing on the target with a fuzzy sight picture 15 yards and in, or focusing on the front sight with a fuzzy target if shooting further than 15 yards). Get your sight picture, then slowly open your left eye. You should stay focused.
I started this a few months ago, and I'm at a point where I can bring my gun up and just see the sights without closing an eye. It's not as clear as with one eye open... I still see two images, but I'm focused on the image my right eye is seeing.
This may sound confusing, and I'm not sure I'm explaining it well, but it's worked remarkably well for me. It does take a lot of repetitions, though.