Get back to the basics, which means get into the steadiest shooting position you can, given the time. Steadying the rifle on a tree limb, or getting into a quick but steady kneeling position (sitting or prone are better) does wonders for the jitters. Deep breath, halfway out, hold, final line-up, squeeze.

I'll bet you took that shot from the standing position with no rest, right? Least steady of all positions.