In Mass Effect 2, I use a strict weapons-based build, focusing around proper use of ammo powers and performance enhancers to deal with each situation.
When I chose an Infiltrator back in Mass Effect, I did so to guide the character as a master of the Sniper Rifle, a focus I have kept in ME2. The Sniper Rifle is your go-to weapon. Powerful and accurate, using it to its fullest purpose is your focus.
There are a few Sniper Rifles available in the game, so always try to secure the correct weapon. The Mantis that you start out with is a very useful tool, but upgrade to the Viper when you can, and upgrade from the Viper to the Widow when you reach the Collector ship. Under no circumstances should you use the Incisor. While the companions can make great use of it, its high ammo cost and odd firing style make it inferior to your original Mantis.
While your Sniper Rifle, especially a fully upgraded Widow, is quite capable of handling your enemies on its own, be ready and willing to use your other weapons as well. Your Submachine Gun is ideal for stripping shields, which is extremely handy on higher difficulty levels for ensuring each Sniper shot does as much as possible, and your Heavy Pistol or Hand Cannon will serve very well when you run out of Sniper Ammo and need to collect, which will happen.
So, on to power loadouts. Your first points should be into your Tactical Cloak and Operative abilities. The extra damage and breathing room these get you cannot be overlooked. Then, start working on your ammo powers.
Disruptor Ammo should not be overlooked. It should nearly always be on your SMG, to help break shields faster, and when fighting mechs, it makes a handy addition to your Sniper Rifle as well. Upgrade to Heavy Disruptor ammo. While the additional damage is nice, the real benefit to this is that you can cause mechanical enemies to explode with the Heavy version, causing each of your individual Sniper shots to do far more damage (especially when Snipe-killing heavy mechs).
Cryo Ammo is next on the list, and this one you will want to make your squad ammo type. When you get into a mission, apply Squad Cryo Ammo to your companion's weapons, then apply your own ammo to your own weapons. Since your own ammo is of the Heavy variety, it will not overwrite or remove your teammates ammo, allowing them to freely use Cryo ammo. This benefits you quite a bit, as any unshielded enemy will be frozen in short order, making them availible to easy kills from any weapon.
The rest of your points are for your bonus power. Armor-Piercing ammo is my favorite, offering and incredible damage boost once upgraded to Tungsten Ammo. Some teams like warp ammo, especially on a Biotic heavy team setup or when facing Collectors, who use Barrier.
When initiating combat, use your Assassination Cloak in combination with your Assassin passive Sniper Rifle targeting slowdown to line up your shots and avoid incoming fire. You should be able to kill most enemies in one hit with a good Sniper shot. If playing on a higher difficulty, use Disruptor Ammo on shielded enemies. Most shields will be taken down with a single Sniper round, and the bonus damage from the shot will carry over into armor and health, including the bonus damage from Disruptor ammo. Otherwise, focus on Tungsten ammo to maximize damage against health and armor.
The main drawback of Sniper Rifles is ammo. While you kill enemies quickly, they drop ammo where they die, which is usually quite a ways away from you. Your Heavy Pistol or Hand Cannon is your key weapon to take enemies out up close, as you move in to pick up ammo for your Sniper Rifle.
Lastly, a word on Husks. Husks are the bane of this build, as they come in large groups, and do not drop ammo. Do not even attempt to kill Husks with your Sniper Rifle, it is a waste of ammo. Instead, switch to your SMG. Outfitted with Tungsten Ammo (on higher difficulties) or Squad Cryo, it is your handy dandy husk killer. Husks die quickly when frozen, or you can shoot their legs out from under them to ensure their swift demise.