What she said!
Seriously, the most common mistakes I see are:
1.) Placing leads V1 and V2 one intercostal space too high on the anterior chest (in some cases even higher)
2.) Placing leads V1 and V2 too far apart (make a "peace sign" with your fingers and you should be able to touch both electrodes)
3.) Placing lead V3 directly under lead V2
4.) Placing lead V4 under the nipple instead of visualizing the midclavicular line
Other common mistakes that aren't related to lead placement are failure to undress the patient from the waist up and leaving a female's bra on. Protect a female's dignity and get her covered up as soon as possible, but don't place leads down the front of a shirt or reach up under a bra. It's absurd. STEMI patients should be completely undressed (similar to trauma) anyway.
Beyond that it's just taking pride in your workmanship. Shave the chest (hopefully for male patients) and use the benzoin tincture. It works! Also, if you're using rectangular electrodes, line up the edges and make it all symmetrical. It helps keep you organized. There's no value in putting some lengthwise, some width-wise, and some diagonal.
Make sure the leads aren't all tangled up. Strand them out individually to minimize artifact. Don't wrap the O2 line, the IV line, the ECG line, and the BP line together. It's a mess, it looks unprofessional, and it's harder to troubleshoot problems.
Be sure the patient isn't holding him/herself up with his/her arms. The muscle tremors will be noted as artifact. The patient should be in a relaxed semi-Fowlers position and breathing normally. You can lay a towel and/or blankets on top once the electrodes are placed to minimize shivering or Parkinsonian tremors.
If you don't settle for imperfection you will almost never have problems with poor data quality (which confounds computerized measurements and interpretations, makes nuanced interpretations more difficult, and harms the credibility of prehospital 12-lead ECG programs).
Good luck!
Tom