remove the handlebar and lightly sand the inside of the stem where it grabs the bar. any debris in that area can cause a slip.
make sure the handlebar clamp area is also nice and clean.
grease the threads of the bolts.
tighten the bolts EVENLY in front of and behind the handlebar.
most BMX stems have some bolt old M8 bolts. you should have no problem putting 12 Nm of torque on those bolts. it's very possible to over-tighten bolts, which can cause the handlebar to get smooshed. it will never really tighten evenly when it gets to that point.