It's /proc/sys/vm/swappiness. As root:
echo "50" > swappiness
will change it, though this won't survive a reboot.
The default is 100, which is the swappiest. You could try lowering it to, say, 50.

You can add a swap partition to your SD card by editing the partitions and adding a Linux swap partition to it (type 82).
For example, add a swap partition of 1GB to the SD. Search the internet for fdisk, swapon, swapoff and mkswap to teach you what to do. You'd turn the SD swap on and the internal swap off.

Please note: this is for information only. Do what you like with your device, but at the risk of making it not work at all. Personally I'd recommend keeping it set up as it is at the moment.