Things change you. Trying to prevent things from entering your soul will only leave you with no direction.
Most of the time people change for the worst. But here's a bit of advice:
Only allow things to enter your soul that act as candle light, brightening the view of yourself, to learn great new things about who you are.
You mostly hear talk about how great friends "changed for the worse". Well, with this advice at leat you can say that your an even "greater friend"... and besides, instead of talking about how far down you've gone, they'll be talking about how high you climbed.