Tip #1: Don't stand out!
First of all, don't stand out and arouse your crewmates' suspicions. That means don't follow potential victims around or wandering about and doing nothing. You'll want to pretend like you're completing tasks, so stand next to computers and engines.
Furthermore, join discussions, primarily when a body has been found. Your silence could lead to your elimination.
The bottom line is, you'll want to look innocent and act as if you want to win the game as a team and eliminate someone.
Tip #2: Act clueless instead of blaming others
You'll want to deceive others into believing you're innocent.
Here's the thing: you can blame others. That will most likely happen, and many of your crewmates will start blaming others, as well.
However, the hard part with blaming someone is that they might have a solid alibi, and you'll end up raising suspicions because of the lie. So, what you'll want to do instead is act clueless. Ask others where they are, what they have been doing, and who they think is the Impostor and agree with them.
Tip #3: Always shut the door!
Your main goal as an Impostor is to sabotage your crewmates' hard work and kill innocent victims--and of course, you need to do it without being detected.
Sabotage their work whenever you find an opportunity, such as killing the lights and actively putting up barriers. Also, always shut the door, especially when you're about to kill a crewmate. This increases your chance of not getting caught and making a speedy escape.
Tip #4: Know who to kill
You'll definitely want to leave the newbies behind and target the veterans first, seeing as they most likely know the game well enough and can quickly pinpoint the Impostors.
Tip #5: Mind who's watching you
Of course, you'll want to avoid being seen killing an innocent crewmate--either by another player or the multitude of security cameras around the map.
You can easily tell if someone is watching the security feed since the camera would flash red. So, always check the cameras if you're about to do something an Impostor would only do--and that includes using the vent.
If you're caught using the vent, it's a dead giveaway that you're the Impostor, especially since it's an Impostor-only ability. Try to avoid using it too much and only use it to escape if you really must. Plus, you should also avoid getting off of vents from open areas, including the cafe or the administration room.