Everyone is different. A lot of people would recommend cutting back on carbs a bit and upping your water intake.
I, however, enjoyed a yummy meal the night before and a big pudding (which actually made me feel pretty sick! Which helped me mentally I guess!). From midnight I only had water until bedtime and then when I woke up I was in slim and save mode!!
If you wanna prep though just get yourself into it mentally - be prepared for a headache, feeling a little chilly and line up lots of things to distract yourself with on the first week.
Good luck on your journey!
I'll be low carbing a week leading up to Slim and save, meat, eggs, vegetables and then switching onto the VLCD. That way I will already be in Ketosis.