Get constant input from the spinnaker trimmer so you can bear off up in the puffs and head up in the lulls. This is especially important for the puffs which aren't always apparent to the helmsman. Lulls are easier to spot because the spinnaker clew sags. When the clue sags head up til the clue rises and you build speed, then bear off. It's a continual up and down process.