Using lftp for mirroring files from a remote ftp or even better http server.

lftp -c "open www.remoteserver.com/directory/; mirror"

Ten parallel transfers
lftp -c "open www.remoteserver.com/directory/; mirror --parallel=10"

Use lftp as download accelerator for a single file
lftp -e 'pget -n 5 ftp://path/to/file'