I don't like bra straps showing under a boat neck and find strapless bras uncomfortable. The solution? Lingerie Stays - little snaps sewn on to loop under bra straps. Your dress stays on your shoulders giving more of a cowl neck appearance.
For help sewing on snaps, here's a short video from Threads magazine. And in two and a half minutes, Susan Khalje from Threads will show you how to sew a thread bar.