Partition the sdcard with the second partition as an ext3 or 4 file system. Would recommend good no higher then 10% for the second partition as any bigger can damage the sdcard.
You do it from from recovery mode under Advance settings. It allows you to use a portion of your sdcard for apps in a similer matter to the internal memory I used data to SD to make my phone actually read it as internal memory, here's a screenshot so you can see what I mean.