A reasonable way is to run an ssh server on your phone.
This does require your phone to be on the net, however.
That is what I am currently doing, but only to copy files from my android phone to my computer.
The SSH implementation I am using is SimpleSSHD.
The instructions on that site should suffice for you to run SimpleSSHD on your phone. It is also on Google Play.
Once you have ssh working, you can use rsync to copy files back and forth.
This appears to be entirely a free software project, is easy to set up, and has worked flawlessly for me so far. The author has included a tarball of his git repository on his web page, but I do not see a link to the actual "live" repository, which is more standard.
NOTE: I'd like to give the author a review on Google Play, but unfortunately Google requires one to create a Google Plus account to do that, which I don't want you to do. If you already have a Google Plus account or don't mind creating one, I encourage you to add a review; there are not many so far.