Hard, just starting the car up as you say will not do the battery any good as you are taking power out & not returning the power, to do this you need to take the car for an hours run, try not to use the Electrics whilst doing this, Heater / Wipers / washers / etc, as said a trickle charge is good, if your battery is of the Top up / Filler Type with Distilled Water ( Yes, there are still some out there ) make sure it's topped above the Internal Lead Cells, other than that, there is not much more you can do, a point, get the Battery Checked before you park it up to make sure the battery is ok.