Dear Dev Juice,
I have a small SSD drive on my iMac. Got some tips on how to offload Xcode to my USB drive instead?
You can install some but not all of Xcode to a secondary drive. You'll still end up using about half a gigabyte, but that's a sight better than the multiple gigabytes you'd normally occupy.
On the Custom Install Screen (right after you agree to become a centipad), set your install location to Other using the folder pop-up. Navigate to your USB and "Choose" a folder there.
After installing, there's more you can do to save room.
Open Xcode Preferences > Locations and set your Derived Data, Snapshots, and Archives folders to live off your primary drive. You'll also want to move your documentation (which can take many GB all by itself) as well.
Open any class reference in the Organizer, and then right-click > Open Page in Browser. Allow the page to open. Its file:// URL will show you where your documentation is currently stored (normally in /Library/Developer/Documentation). Use symbolic links to move that material to the USB drive.
There are more tweaks you can do (move your simulator home, mess with ~/Library/Developer as well as /Library/Developer) but they tend to produce fewer gigabytes for greater effort, so aren't recommended.