answer is sort of recursive, you need to make transactions with other important entities

to counter upcoming question:
"what if you would have 5 accounts that would send nems between them to boost their poi?"

it won't be possible to artificially boost PoI like that, due to vested balance and one other mechanism (that we'll keep secret for a while).

So let's assume we have 10 attackers, who would like to boost their score by sending txes to themselves.
The thing is they would actually lower it. And since PoI is sort of constant for network as a whole,
attackers would additionally boost a little PoI of everyone else