Why does my cigarette lighter keep blowing fuses?

Hi ~
About a year ago, the fuse for my cigarette lighter blew (2 cigarette lighters underneath radio in front)...I replaced it and it worked fine for about 6 months...then it blew again...the next one lasted about 4 months...the next one lasted 2 months...and just recently I had one blow after about a week and then one blow after only 1 day!!

They are used mainly for cell phone charges and ipod fm transmitter...

I realize its not going to do any good to keep replacing the fuses as there is a problem somewhere....can you shed some light?? While I can survive without my cell phone charger...my son cannot survive without his ipod music!!! :)

Thanks!