Reply to Mixing with an MP3 player

Hello!
I'm just starting out in Djing and from what i understand a lot of digital dj's use an MP3 as a back-up plan in case the laptop crashes....

I know how to use it with my S4 controller...but i wonder what most of you did (Hopefully not) or plan to do in case you have to use it at a gig.....Pre-made sets? Or is there a way to mix/make a somewhat smooth transition between songs in an MP3 player which i'm unaware of...

THANKS IN ADVANCE!