If you do a lot of short trips and have a drive way (or a quiet street) get an intelligent battery charger and hook it up every few nights...
Maplin also do these..
http://www.maplin.co.uk/module.aspx?ModuleNo=98358&doy=17m12#overview
dont forget though that the cig lighter is isolated with ignition off so you will need to clip it to the battery directly...
Not enouth sun light i belive
plus seems like a bit of orginisation to remember
I gave it a very good drive today