How can I play Infiltrator effectively?

What you're dealing with is basically just the fact that it's a slower payoff than a melee specialization.

Sticking with the cloaking and sniping tactics will payoff. You'll have to invest a lot into building your rifle up, but eventually most enemies will be one-shot kills unless they have multiple layers of shields.

Of course there is a lot more in the way of hybrid options in ME3 than there was in ME2 so some of it will be up to you and how you play the character.

Cloak + Shotgun = AMAZING!

Right now my load out is: Mantis V Wraith V Shuriken V

Cloak and Operational Mastery are at 6 and Fitness and Disruptor Ammo are at 5. Everything else is at 1 until I max the others out.

Focus on getting your ammo powers up and your Tactical Cloak as well. Also make sure your putting points into fitness for the incressed health and shields. Only melee if you are flanking and only if it gives you a good postion to take cover after you decloak.

I would recommend trying to unlock the Quarian Infiltrator to gain access to Sabotage. It's the equivalent of the AI Hacking ability in the previous games and there have been numerous threads popping up recently about how powerful the effect is since you can use it to turn Geth Prime and Geth Pyro enemies to your side temporarily. You don't even need to deplete the enemy's shields before using it on them.

I believe that the fine gentlemen of the Beastie Boys said it best when they said

Listen all of y'all it's a sabotage

I play my infiltrator as a techy. Sabotage, cryo ammo, incinerate, specced to stack damage and do combos. And using the two powers thing with cloak ability. No need for a sniper just use the tempest 5. High rate of fire freezes EVERYTHING with cryo ammo and allows for easy triple damage incinerate. Sabotage also doubles this if it precedes them. Then maxing out your gear for power speed and damage. This hits like a truck, but i suggest palling around with EDI for her overload. Very useful.