Nothing. I've left mine for three months without doing anything special. A lay-up for much longer and you'd might want to consider over-inflating the tyres and using a battery conditioner, but 2 -8 weeks is nowt.
Current: 2014 F31 320d M-Sport, Professional Media, Visibility Pack, Reversing Camera, DAB
Previous: 2013 F10 520d M-Sport, 2011 F10 520d M-Sport, 2008 320i M-Sport Coupe, 2002 325i, 2001 318i valvetronic, 1998 318i, 1996 525i, 1990 Porsche 944S2