Already a lot of good advice given here. When I'm chasing a grain, I'm back and forth from using a drawknife to remove the bulk of the wood, to dressing it up with a scrapper and reconfirming where the transition line is between one grain and the next.
But like George said, that's just a technique for chasing the grain. To follow the lateral grain for laying out a center line, cleaning up the remnants of the soft honeycone wintergrowth with a scrapper, and then lightly with some sand paper full length of stave will help the lateral grain show up with good light conditions.
Before I actually start with the centerline, I try and make a game plan on what type and length, etc. bow I can best make with the particular stave you've got. If you have a 75" stave, and want to make a 68" bow from it, ask yourself where best to locate the handle such that the limb tips will allow the string to lay down the handle center. Mark that handle center, then locate the other two ends of your bow 34" off center. Know that you can always tweak bow limbs with dry heat or steaming to align if need be. Then I layout the centerline down the stave and establish limb/handle widths off that centerline. Slightly tweaking a line to either side to include or avoid a pin knot or some other feature can be done if you don't violate the lateral grain to much.
Like already mentioned, the more of this you do, the better you'll get at it.