Before I start speaking English I sing the 'Hello' song. It is taken from the BBC programme called Something Special. (Jump to 0:20 for the song)
Before we change into Hungarian, I sing the "Goodbye song" from the same programme. (You can find the video below - just jump to 13:20. This is where the "Goodbye song" starts. Otherwise it's a great episode on birthdays. Worth watching the whole). I also use the MAKATON signs while I sing. E. takes pleasure in it and the songs make it clear when we use one language and the other.
I'm sure there are several other ways to give a signal for changing between the languages. I look forward to your comments on how YOU do it.