I notice people taling about this problem a lot on blogs during the summer months, what with all of the sleeveless tops and dresses, and I thought I would do a tutorial on how to get rid that that excess and put it in the bust dart. (You can just make an armhole dart, but I like to move that dart into the bust dart. Just to be tidy.) Since the tutorial was so picture heavy, I decided to upload it onto the burdastyle website. (Also, I just wanted to try their tutorial format.) You can check it out here.
Look Ma, no gaping armhole!