Blackjack Strategy When Playing a Soft 18
Double Down: When the dealer upcard is 3, 4, 5 or 6.
Hit: When the dealer has a 9, 10 or ace.
Stand: When the dealer has a 2, 7 or 8.
Why to double down on soft 18
The strategy for a soft 18 is more complicated. When the dealer has a 3, 4, 5 or 6, the odds are that the dealer will eventually bust anyways. The soft 18 is also a pretty decent hand to hold so you have two advantages working for you, therefore it is ideal to double down. The basic strategy is that there are more tens in the deck of blackjack than any other card so always remember this fact.
Why to hit on soft 18
You will want to take a hit when the dealer has a 9, 10 or 11 (ace) showing. Since there are more tens in the deck, the dealer will have a very good chance of beating an 18 here. Recall that a soft 18 will not bust so you have more chances to make a better hand because of the ace in play.
Why to stand on soft 18
This strategy is not so obvious but it lies mostly with the mathematical edge over the long term and the blackjack basic strategy that says the dealer will likely get a 12, 17 and 18. The 12 would likely go bust, the 17 would be beat and the 18 would result in a push. The most important fact is you wouldn't lose the bet in this case.