Probably been said a few times but I think the most common ways to ease into a tempo change is drawing out a note on the last line, then maybe a crescendo with the voice to show climax and annticipation of the ensueing intensifying of the song then BOOM! Or before the boom, maybe the old classic chugging downstrokes on the rhythm guitar, then a bass line then a nice little riff on lead (awesome for a solo musician on a loop pedal), all adding to the climax bar by bar...DAMN that shit makes the breath rise in my throat. Shivers. Gotta love a tempo change, no one forgets that shit soon.