The best strategy for doubling down in blackjack involves assessing both the player's hand and the dealer's visible card to make informed decisions. Doubling down is most advantageous when the player has a strong total, particularly 11 or a soft 16 to 18, as this maximizes the potential for a high-value hand. Additionally, it is optimal to double down when the dealer shows a weak card, such as 2 through 6, which increases the likelihood of the dealer busting.