bratricks Make a strapless bra stay using a removable bra strap. Put on the strapless bra and clip a strap to one side. Wrap it around the front of the bra at the band level and attach it to the other side.
If you need a life hack though here are a lot. I haven't tried or researched each tip to verify if it works so bear that in mind. However, if you tested any of them and know that it works, please confirm in the comments.