When I started doing LARP, I had been playing pen&paper RPGs for well over a decade already. LARP taught me something important about immersion.
In LARP, you are your character for the duration of the game. That helps immersion. It also makes you notice immersion breaks much more strongly.
What breaks immersion in this context and is applicable to pen&paper?
- out-of-character events, discussions, etc. -- arguing with the GM, real-world-disturbances, rules mechanics
- out-of-the-world things -- cars in fantasy LARPs, jokes that don't work in the setting, characters showing knowledge of modern chemistry in a medieval world, etc.
- modern stereotypes and biases - or lack thereof -- racial and gender equality in a medieval world. Or the reverse - racism in a SciFi world (when all evidence points to racism decreasing as mobility increases).
The main thing that you can control as a GM are the meta-gaming elements (rules discussions, arguments with the GM, etc.) and the setting-breaking-stuff.
One: Make sure your setting is portrayed correctly. Do some legwork and put some effort. Let the players experience that the world is different. Don't shy away from racism and sexism if it is appropriate to the game world. Make it clear before the game that you as a person outside the game are not like that, but that the world they play in is different from our world.
Two: Remove rules discussions from your game. I typically have a rule in my game that if nobody knows the exact rule to apply or roll to make in a given situation, I as the GM will make something up on the spot and we will use that.
Third: GM word is final during the game. If anyone has a problem with my imprompto rule from step 3 or with any other decision I make, he can make a one-sentence argument during the game. I'll decide to accept it or not. Any extended discussions or looking up of rules happens after the game or during a longer break if there is one (e.g. for eating).