Position: Your head and body must always be in the same place for each shot. Target shooters use the "natural point of aim", where you get into position with the rifle pointed at the target, close eyes, shake rifle, and the sight picture should be dead on. This assumes you are shooting from position, and not from a bench. A lot more can be said about position . . .
Sight alignment: look through the center of the rear sight. I place the bullseye at the center of the rear sight (some place the top of the front sight at the center).
Sight picture: for bullseye targets, I prefer a modified six o'clock hold, where the top of the front sight is a sliver of white below the bottom of the bullseye. It looks kinda like this: i. In the regular six o'clock hold, the top of the front sight touches the bottom of the bullseye. Use one of these two sight pictures (there are others, but I don't reccomend them).
Remember, your focus should be on the front sight!