How to save text messages on blackberry bold 9900?

If i save them to my phone,they get deleted month by month and if i save them to my sim card it tells me i have to delete some messages because theres not enough space...

how do i save texts in general? not where you specifically select a message to save..but all my text conversations in general?

edit-
the only options i have in my messages settings is to either tick or untick the 'leave messages on SIM Card' option..

You can set all messages to never be deleted. I have messages still from way back when I was using my Storm 2 in 2008, and it's still carrying over onto my 9860. It's under messages, then settings.
OR,
You can back the whole thing up, and use magicberry to export the texts into txt format