One of the primary uses for setting screens (press and hold the B button on offense) in this game is to get a great shooter open beyond the three-point arc. The long range shot, or inability to hit it, decides many games. That's especially true in online play. A recent Xbox Live update is supposed to remove a glitch that makes step back three-pointers an instant shot. But a couple of quick three pointers can still turn the tide of a game.
A lot of players have a specific point on the court they're most comfortable shooting from. You should plant a defender at that spot the second the opposing team begins setting up. And don't be afraid to intentional foul with the Back button. Throwing some fouls in early is a good way to check the opponent's free throw skill. If they miss even one of three shots on occasion, fouling them becomes an infinitely better option than letting them fire up uncontested threes. Also, on free throw attempts you get the option of pressing X and A to make your opponent's controller vibrate. It may throw them off and it may not, but it's an option you don't have while they're shooting a three.
If you're not the best long range shooter yourself, it's time to hit the practice gym. NBA Live 10 does a good job of imitating the signature shooting styles of the world's best b-ballers. That means launching with Kobe isn't the same as Peja Stojakovic's odd looking but effective fling at the bucket. Practice at length with your favorite players so you get a feel for their shot and the moves they'll use to get open for it.