Okay, you think you play a decent game. You are pretty good at basic strategy and generally play as most other players. Or worst of all you deviate based on hunches or feelings. Let's take a look at what that costs.
You don't hit your 16 vs dealer 7 
costs 

You don't hit your 12 vs dealer 2 
costs 

You don't hit your A7 vs dealer 9 
costs 

You don't hit your 16 vs dealer A 
costs 

You double your 10 vs dealer A 
costs 

You must play perfect basic strategy if you are to have any chance at all.
Get the picture. The fact that most people don't tells me that they look at the basic strategy chart and their eyes glaze over. Mine did too.
Then I organized the chart below and learned it like I used to learn phone numbers. Remember those days. Cell phones have freed up brain cells for other uses. Learn each line like a phone number or a line from a song. Today if I see a 12 anywhere I automatically think "stand 46 otherwise hit".
The Average Player plays at a 3% to 4% disadvantage to the house
Blackjack Strategy Chart:
Universal Strategy for all Decks and Rules
The only Blackjack Strategy you will ever need.
Click here to purchase convenient walletsized plastic blackjack basic strategy card.
Basic Strategy Table
Player
has

Dealer shows... 
Below 9 
hit (always) 
9 
Double Down when dealer shows 36 otherwise hit 
10 
DD except 10 and Ace otherwise hit 
11 
DD except Ace 
12 
Stay 46 otherwise hit 
13  16 
Stay 26 otherwise hit 
17  21 
Stay (always) 
Ace, 2  Ace, 3 
DD when dealer shows 5, 6 otherwise hit 
Ace, 4  Ace, 5 
DD 4,5,6, otherwise hit 
Ace, 6 
DD 36 otherwise hit 
Ace , 7 
Stay 2,7,8, DD 36 hit 9, 10, Ace (most misplayed hand in blackjack) 
Ace, 8 Ace, 10 
Stay (always) 
Pair A,A or pair 8,8 
Split (always) 
2,2  3,3  7,7 
Split 27 otherwise hit 
4,4 
Split 5,6 otherwise hit 
6,6 
Split 26 otherwise hit 
9,9 
Split except dealer shows Ace, 7,10 
10,10 
Stay (always) 
Never take insurance or even money 
Click here to purchase the above table. Credit card size and made of sturdy plastic  3 for $0.99 cents
Some casinos allow blackjack tables to be used openly, others don't. When playing online keep this table at your side.
Daniel is now selling some Basic Strategy Tshirts in his store. The Basic Strategy chart is upside down on the front of the Tshirts. One glance down will remind you how to play your next hand.
The back is rightside up for your neighbor. Oh, and note the cool basic strategy coffee mug.
These Tshirts have proven to be instant icebreakers! It will make you the center of attention. Great for meeting people and oh, it will help your game. 
You have an Ace and 7, the dealer shows a 6. What do you do?
If you didn't immediately say Double Down, you need this Tshirt.
Don't have time to study basic strategy? Daniel's got you covered. Click to go to Daniel's store
Play and practice for free
Practice is the key to success in all fields. Several students that I have trained to become professional players all used this practice method. It simulates gamelike conditions. First copy or purchase the basic strategy chart found above and keep it by your side. Click on the link to go to 888Casino. Click free download. A page of standard regulations will appear. Read and click agree. Choose a currency. You will receive a user name and password. Save them. This all should take a minute or two.
Now click on play for free. That's it. You can practice all you want at no cost. Go at the speed you are comfortable with. Leave and return whenever you desire.
Test yourself. Play 20 hands without looking at the chart. Click on history and the hands you just played will appear in a neat column. Go down the column while checking it against the chart. Not only are you learning but is great fun at no cost.
The above basic strategy is generic and serves me well for all games. The gain from learning a slightly different strategy for single and multideck is so minimal that it is not worth the effort.
That said I offer the following:
If Surrender is available:
Sr 16 vs dealer 9'10'A
Sr 15 vs dealer 10 
If DAS, double after splitting, is not permitted:
do not split 6,6 vs dealer 2
do not split 4,4 vs dealer 5 or 6
do not split 3,3 vs dealer 2 or 3
do not split 2,2 vs dealer 2 or 3
What are you doing playing a game that does not allow DAS anyway? 
In a single deck game
stand on 7,7 vs dealer 10
and stand if you draw out 4,5,6 to a dealer 10
The above should be obvious in that you have used up the cards that could help you. You are in a bad situation and it is best to hope for the dealer to bust. 
Well now that you have seen basic strategy it is important to learn it. To learn anything it is easier if you understand it and believe in it. I don't want to go too deep into the details but the following explaination may help you see some of the reasons for the above basic stratgey chart.
Players have the most trouble with the A, 7 vs dealer 9, 10, A.
An 18 appears to be a pretty good hand and many people are reluctant to hit it. They reason why possibly ruin a good hand. The answer is that it is not a good hand against those dealer cards.
Look at a dealer 9. There are 13 possible cards under that 9. The ace gives the dealer a 20 and four chances for a 10 gives the dealer a 19. So right at the start you have lost five times. A 9 gives you a push and only an 8 a win. So with seven possible cards you have five losses, one tie and one win.
It gets more complicated now. If the under card is a 2 the dealer has an 11 with four chances of drawing a 10, one chance each at a 9 and 8 which beat your 18. In short the dealer will make good hands on about half of his draws resulting in the player losing 7.5 times, pushing once and winning only 3.5 times.
Now let's hit your A, 7 as basic strategy requires. An ace, 2 or 3 improves your hand and a 10 leaves it the same. If you draw a 6 for example and make a 14 you can hit again. You will make a good hand on about half of those draws. In summary look at the following table.
If you fail to hit A,7 vs dealer 9 your loss is 
8.00% 
If you fail to hit A,7 vs dealer 10 your loss is 
4.00% 
If you fail to hit A,7 vs dealer A your loss is 
0.37% 
If you fly on an airplane, cross a bridge or ride an elevator you trust the math. Winning blackjack is all about math. Trust it. Hit your A, 7 vs dealer 9, 10, A.