This method might not be the easiest one out there, but it works. Heres how you do it.
Things you need:
1. A rooted phone.
2. A partitioned memory card (both should be primary partitions, file system of second partition should be ext2, ext3, ext4, fat16 or fat32)
3. A free app - Link2SD
Well, I'm just saying the basics, the details about moving an app to SD are explained both in the app page and in the app itself. The second partition is used to store apps, and it stores the entire app, where as when we usually move an app to SD card, only a part of the app actually goes to the SD card. Once you've installed Link2Sd and formatted your SD card properly, open Link2SD, press the menu button, select settings, select "auto link". Now all the apps you install will be installed in the second partition.
This way, you can even install apps by default to SD, even those that cant normally be installed to SD!
If anyone is having trouble rooting or using Link2SD, let me know, I will try my best to help