sftp the easiest way is probably to use the ⇧ arrow to return to previous command, then Home or Ctrl+A to go to the beginning of the line, then L Enter. That is:
For your use case perhaps the internal SFTP client is not the smoothest way, though. You don't mention OS, but if it concerns *nix, then using an SCP wrapper script directly might be a simple way to save time in the end, creating e.g.
mydownload that attempts to upload or download the file given as argument to/from the server respectively, automatically deciding the correct paths from some given base project paths. Set up key based login so you don't have to enter a password every time.
Another pro with using SCP from the command line on *nix as compared to the SFTP client is that you will be in a familiar shell environment, most likely Bash, and have access to regular navigation tricks among the commands such as tab completion, command repetition, brace expansion, etc.
Personally I prefer version control systems for these kinds of tasks. It will give you simple uploading using e.g. a commit hook, and version control is very handy for code projects. I don't know exactly what you are doing so it might not fit perfectly, but if you like, take a look at e.g. Mercurial or Git.