Here are a few tips I have gleamed from this nasty experience:
1. The second that you cut out the sleeves, use chalk to mark right/ left sleeve, which side the vent placket will be sewn, any button holes and pleats of course, and if your fabric has no wrong side, designate one. (Tip courtesy of Mom, who once made the same mistake.)
2. Take a good look at a shirt already made. Note how the sleeve is shaped, where seams rest, placket placement etc and compare it to your pattern. I drew myself a diagram for future reference.