I love these simple long sleeve undershirts for layering.  Gerber does run small...my daughter fits into her size by age, but she's in the 1% for weight and 30% for height and at 7 months, she's almost outgrown the 6-9 month size.  Gerber makes onesies for long narrow babies, so if you have a particularly chubby or short torso baby, you may want to try a different brand.