I have successfully made backups of a TrueCrypt encrypted system drive using "dd" - the block copy utility found on unix-like systems.
http://en.wikipedia.org/wiki/Dd_(Unix)
With dd, you can clone your TrueCrypt drive to another drive, or you could write the entire contents into a file. Please note that if you write to a file, the file will be the size of your entire TrueCrypt volume. E.g. If your TrueCrypt volume is 200GB, the image file will be 200GB, even if only 40GB of the TrueCrypt volume is actually occupied. This is because you cannot tell the difference between free space and used space on a properly encrypted volume.
The actual command you run would look something like this:
For clone:
dd if=/dev/sdX of=/dev/sdY bs=16M
Where "sdX" is the drive you are reading from, and "sdY" is the drive you are cloning to.
For backup to image file:
dd if=/dev/sdX of=/path/to/file.img bs=16M
Where "sdX" is the drive you are backing up, and "/path/to/file.img" is the image file destination.
These commands will of course also work fine with a non-encrypted volume.
The "bs" parameter is just for "blocksize" and will help speed up the copy process.
Please be careful not to switch the "if" (input file) and "of" (output file) parameters. There's no safety mechanisms with "dd". It won't ask if you are sure you know what you're doing, and it won't stop you; it will just do whatever you ask.
I'd recommend running the backup operation using a Linux system. Even if you don't use Linux normally, you can just boot up a copy of Linux from a Live USB memory stick on the computer where your drives are attached, and make the backup.
Edit: To address your question about compression - You cannot compress an encrypted volume. If you find that it is possible to compress an encrypted volume, that means your encryption implementation is not secure.