Just using iwconfig to set your rate, it will revert after rebooting.
You can add the line to your /etc/rc.local to make it run every reboot though.
I prefer to use something like:
which is "normal auto-rate up to 18M"
iwconfig wlan1 rate 18M AUTO
There are a few other things that can help with unstable/low quality connections...
namely setting RTS & fragmentation threshold (default 2300ish) to 512 or lower to force smaller packet sizes (down to as low as 256). With this you could get stable at much higher data rates than before.
Overall, I start with this to my /etc/rc.local & tweak occasionally til it's rock stable.
iwconfig wlan1 rate 36M auto
iwconfig wlan1 frag 512
iwconfig wlan1 rts 512