Driving short distances with heating and audio on tends to put strain on the battery diverting the current that is supposed to charge the battery into running these items. firstly, get the battery checked and fully recharged if low. also, get your alternator checked. most places (electicians) check it for free. if you can't find an electrician, drive the car around for a while then when you get home, disconnect the battery by removing the terminals. leave it like this overnight and reconnect in the morning to see if the battery starts the car. if if kick up with vigor, then the battery is ok and it is possible that there is a current leak somewhere in the cars electrics ( Not good news). other than that, you can either invest in a new battery or pick a good one from a reliable scrappie for less than a tenner.
you could also ask an electrician for the power output of your alternator and get a battery just lower than that power, meaning if your alternator puts out 50Ah, you could get a 42Ah or 46Ah battery instead of the normal 36Ah. your alternator will still charge it but it will have more power. just make sure the battery fits in your engine bay securely.
Also, if you make short runs, try not to use your audio/unnecessary electrics to reduce the strain on the battery.
The best way still to check your battery if you don't want to go to an electrician is to drive around till you feel its charged(about a 20 mile trip), then disconnect the battery and reconnect it in the morning.
with summer round the corner ;-), you could invest in a solar trickle charger if you don't use your car that often.
Else, go to a car boot sale and pick up a battery charger and charge your battery every weekend to keep it topped up.