The polygraph isn't about lie detection. If you can't beat an NSA poly, they wouldn't want you. The poly isn't about finding 'truth' inasmuch as it's about your abilities to pass it. I can't speak to the NSA specifically, but in other agencies, the poly is a screening tool. Even if you're lying but can beat the poly, then you have proven that you can maintain while under extreme pressure. Intel agency polys are seriously stressful. Often 3 days long, sometimes administered in a generic northern Virginia chain hotel room with constant badgering. The goal is to maintain physiological control under mental duress. Polys are worthless as lie detectors, but they are a good test of mental toughness (or sociopathic skills.) the easy part is just to tell the truth. Unless there's something in your immediate past that's a disqualifier, usually within the previous 12 months, then there's nothing to fear.

If you're easily rattled, even if telling the truth, the poly weeds that out.