Jamie over at Proper Bikes posted this on their blog earlier this month. Reading through it, I have to say that I found it very accurate and to the point. That’s why I decided it is worth spreading the message some more.
This was posted as some do’s and don’ts for their stems hat are about to hit the market, but seeing that 100% of stems have bolts and threads in some form, this goes for all stems.

• Make sure there is steerer tube under both of the rear allen bolts. If you cut your steerer too short then don’t crank the top allen bolt up as hard as you can and wonder why your new stem doesn’t like it.
• Tighten the bolts evenly. That’s right, you need to clamp the bolts (the four front ones and the two rear) with a similar amount of torque. Don’t try to achieve this by cranking one allen bolt after the other as hard as you can.
• Don’t use a 6ft extension bar to tighten the allen bolts. A 10cm long allen key will supply enough torque to keep your stem tight.
• Don’t spray your stem (or bars) and wonder why either slips.
• If your stem starts to distort in any way you are doing something wrong. Do not continue to tighten your stem at this point.
• Once your stem is plenty tight enough to grip your bars and steerer tube (whilst a hippopotamus performs a footjam whip or gaps to flat from numerous storeys) there is no need to put in a few extra cranks ‘for good luck’.