Sit 'n' go, single table tournaments of SNG whatever you want to call them are a great way to learn poker and build a bankroll from a minimal deposit. Some of the world's best players, including Tom "durrrr" Dwan, began their careers by playing micro and low stakes SNG before moving onto the high-stakes cash games they now play.
Whilst it is true that some very intelligent individuals have essentially "cracked" SNG play, that is analysed millions of hands and situations and come up with the mathematically perfect way to play, there is still plenty of scope to make money from the lower end of the buy-in spectrum, where the weaker players continue to make the same mistakes over and over again.
One of the most common mistakes you will see at the tables is players limping too much. This is most common when the blinds are low because the limper thinks that they have the potential to flop a big hand and double up, but what they fail to realise is they are leaking chips, chips that are vital in the latter stages of the SNG and chips that are costing them much needed equity. Taking the initiative will allow you to continue the momentum after the flop and will make you a much tougher player to play against.
Following on from limping too much, another major mistake is to not attack passive players. A passive player will hardly ever raise preflop and will have some strange looking HUD states (if you are using one) that read something like 45/8 or 35/7. These players will almost always call your preflop raise and then check-fold the vast majority of flops, quite literally giving you their chips. These players are more common than you think and you should attack them relentlessly, that is until they start to play back at you, which they will only usually do with the most premium of hands.
Although the "perfect strategy" for SNG is widely accepted as playing ultra-tightly in the early stages and then going nuts late on, it does not mean there is no scope for some creativity. Because it is so easy to play multiple tables of SNG simultaneously, you will often come across regulars who are playing several tables at once. These will be playing the aforementioned robotic style of playing very tight to begin with and loose late on, perfect to target during the early stages and grabbing some much needed equity from the off. That is not to say play like a man possessed, but you can open your range up against these regs as they will be spending the majority of their time folding whilst the blinds are small.
The biggest mistake SNG players make are always around the bubble, that is, the last position before the money is awarded. You need to familiarise yourself with the concept of Independent Chip Modelling (ICM) and know how to apply it. In order to fully utilise ICM, a player needs to be able to put their opponent on a range of hands accurately, which comes with practice and experience. Once you delve into ICM, and possibly use an interactive tool such as SNG Wiz, you will be amazed at which hands you should be folding when facing a shove and which ones you should be pushing all in with when first to act.