Lightly sand the streaky areas with fine grit paper until smooth. Then apply a fresh coat of stain to the entire board or section, not just the spot, to blend it in. Work with a fully loaded brush and keep a wet edge to avoid lap marks. Doing smaller, manageable sections will help you maintain control and get a more even finish this time.
I always use 220 grit for that final light sanding before the last coat. It knocks down the streaks without cutting too deep into the previous layer. What grit do you usually reach for?