Relax, and let the line come...usually it does so only exactly at the time when you need it. And then there are the instances when it does not, and you make a joke, and wait for the music to come around again, and relax a little more, and hope like hell it does not happen again.

If it does, apologize, move on with it, and practice the piece all the more before trying that again.

At least, this is what I do! :D