i have wireless working good. I want to be able to connect to other wlans too but i have not found a way to do that yet. For example when i take my laptop to my friends house and i do a iwlan wlan0 scan i detect his wlan . It is insecure etc but thats ok for now.

Cell 02 - Address: 00:12:25:55:62:59
ESSID:"sv12"
Protocol:IEEE 802.11b
Mode:Managed
Frequency:2.462 GHz (Channel 11)
Quality:0/100 Signal level:-86 dBm Noise level:-256 dBm
Encryption keyff.

Thats the wlan i want to be able to connect to .

If i just do iwconfig wlan0 etc it will overwrite my /etc/networking/preferences file that i need for my wlan at home which i dont want to happen. I can backup my preferences file and make a new one for the above wlan but thats a hassle having to back up that file all the time

any tips?

Running iwconfig on debian doesnt overwrite any file that I am aware of. I would just do
iwconfig wlan0 channel 11
iwconfig wlan0 essid "sv12"
iwconfig wlan0 dchp
unless the ap doesnt do dchp. then you need to set it up manually.