To me, Infiltrator seems like the easiest class to blitz the game with. Once you get the M-98 Widow sniper rifle, you can kill most enemies with just a single headshot.

Which ever side you choose, paragon or renegade, stick with it throughout the game. Also, you should do Jack's loyalty mission last, because the post mission conversations will require pretty high karma. She's a complicated being and the only one that can be only partial loyal after her mission, which can result in her death at the end of them game.

If you banged Ashley in first game, don't cheat on her in ME2 if you are planning to play ME3 lol

Most bosses are heavily armored, this is where armor piercing round comes in really handy.

Credits are limited, so don't spend it aimlessly, only buy upgrades that would help your squad combo.

When you hit the last mission, you will want to look up a guide on how to split up your teammates. Bad decision can result in their death or yours.

Which also reminds me, if you die in ME2 last mission, you won't be able to use that game save in ME3 since...you died.

If you choose Morinth (the seducing daughter) over Samara (the cougar mom), don't eat the forbidden fruit! (if you know what I mean)

Thane, Legion, and Tali sucks

That's all I can remember for now.