I must have mentioned this before, but as I sewed in sleeves yesterday it occurred to me that this should be said again - pin, pin, pin when preparing to sew garment pieces together.  More fullness to ease in, more pins.  And just as important, remove those pins as you sew so you don't bend or break a needle sewing over them.  Be sure your pins are sharp so you don't snag your lovely fabric (discard the dull ones).