What to send trough sockets (cross language)

I have a project at work, where i have to use a server-client environment, my daemon i would like to write in Python connected to a local MySQL database and should handle all the requests of the clients, the clients would be also python for serial barcode reading, en sending to the daemon, but also a management application in .net that could execute commands and/or request data from the daemon.

Now, i managed to send text to and from the clients to the server, (very cool btw) but what is the best practice to send commands, recieving data (fetched from mysql) etc...?

I see alot arround XML / SOAP etc..., is that the best/correct way to send data?

how does other applications this? like an exchange server for example? (just curious :))

Many Thanks for your advices!