You may also be able to use the status lines in RS-232, although synchronization may be an issue.
Brief:
RS-232 was made for computers to communicate with modems, so there is a number of
status lines available: RTS, DTR, CTS, DSR, RI, DCD. The first 2 are computer to device, the last 4 are device to computer.
Depending on the API available, you may not be able to accurately detect when exactly the status lines change. If this is the case, you will need to put breaks (technical term) in the data. A break is simply when no data is transmitted for a while (ie, not continuously).
Of course, if you're just transmitting data, and not using full RS-232, this will be of no use to you.
Another option would be to use 9-bit or larger words (word=a single transmitted frame). This may or may not be available depending on your hardware.