Another solution that works for a lot of medium-security users is to just create AES-128 encrypted disk images in which you can put the files. Then you can mount the disk image and manipulate files in it, unmounting it when you're done.
I do this, and recommend it, with one warning: don't rely on a single encrypted disc image. Instead, make lots of disc images, each no larger than a DVD [let's say 4.1GB, to err on the safe side]. Don't try to get away with only using one.
One day, you'll be writing to your encrypted disc image, while it's mounted [and thus, not encrypted at the moment]. The power to your computer will fail [even happens with MacBooks and MBPs, folks]. At that moment, all the data in your encrypted disc image will be destroyed. I'm not saying this *might* happen. One day, it *will* happen. Judge for yourself what's worth the risk.
Same thing can happen to you if you're using FileVault, but at least with separate encrypted disc images, you can only lose what was in that particular image. If you're feeling extra cautious today, you can copy a .dmg file before you mount it, do your work on the copy, unmount it, and then decide to keep it and trash the original.
A few years ago I archived some files downloaded off the internet, all onto an encrypted disc image that was going to be filed away where the kids couldn't get at the files. We had a brief blackout while that disc was mounted. It never opened or functioned again. I was using the Move command to move those files, so they were erased at the source -- files lost forever.
Next time around, I put the files in 4 different .dmg files, and I *copied* instead of moving them. If the power had blipped during that process, I'd have only lost a portion of the data.