Short answer: You can't.

Long answer: There are three options:

• Since a card with a FAT (Windows-compatible) filesystem can't store UNIX permissions, you can move large data files to the card, and symlink them in; but the apps themselves stay in memory.
• You can put an ext2/ext3 filesystem on the card, which will render it non-readable in Windows, but allows you to move entire directory trees, like /usr/, onto it.
• The best option for most people is to repartition the card with a FAT partition for file access, and a ext2/3 partition after that; you can then clone the entire internal memory into the (larger) ext2 partition, and boot from the SD instead of main memory.

While cloning and dual-booting is the best of these three for most people, it's not without it's difficulties. The best thing to do if you're uncomfortable with that procedure is to clear out the demo video and user manual PDFs (at least the ones not in your language!) that come on the device, and maybe keep your own data on the card instead of in main memory; you can usually clear up enough space for the apps you need that way, unless you're doing seriously advanced stuff.