Using a delimiter to mark end of packet. For text protocols we usually use "\n" to be the delimiter. This usually requires some escaping in the packet to make it unambiguous. Two standard protocols for this are SLIP and HDLC.
Length encoding - like what you did.
The downside of the delimiter approach is that it changes the length of the packet - when you are escaping one byte becomes two and that's sometimes a pain if you're doing it in place in memory (less of a problem if you're streaming byte by byte). The big advantage is that it allows for resynchronisation - if you lose a single byte from your stream, or your length byte ever gets corrupted then you're permanently out of sync - the receiver will never again know where the start or end of a packet is. With the delimiter approach, you just lose one packet. So if you're ever doing this for a UART or network stream or something, always do the delimiter approach!