During the summer season I have purchased a couple bat wing shirts with large arm openings OR shirts that are more on the sheer side. 

Typically I would wear a tank top on the bottom but...

With Sheer tops....

1) Its hot enough during the summer already, adding another layer would not help. 

2) Sometimes tank top can ruin the "flow" of the top and make it not as fitting and flattering. 

With Batwing-ed shirts...

1) When you lift up your arms, your bra is revealed to the world which I don't think is classy.

My solution? Bandeau! Not just normal bandeau but laced ones! I am absolutely loving the lace and the look it gives. So feminine and delicate. I have seen some at Urban Outfitters that retails for $14 a piece which I think is a bit pricy for a bandeau. The ones that I got was from Wet Seal and its only around $4 ish dollar a piece!