I am currently designing an application that will require data transmission. I am currently working on the client software that will build the data packages that will be sent via the network level service. 
What data type should I use for network transmission? I am currently pondering whether I should use a physical data file (.dat) which can be easily manipulated (created/read/etc.) via  Python or use only internal data. From a management and organizational standpoint, I think file based data may be the easiest to manipulate and handle on a networking level. 
If I were head more towards a internal (Python) data handling method, what should my starting point be? Should I look at dictionaries? The over-arching goal is to keep data size minimal. Using file-based data, I believe I would only be looking at just a few bytes for actual transmission. The native platform is going to be Windows, but I would also like to look at my options for a mobile standpoint (Android/iOS).
The purpose of the program is data entry. User entry will be recorded, packaged, encrypted and sent (via a WAN) to a server where it will be stored in a database for query at a later time.
There are best practices, definitely.
As a first advice, you should definitely decouple the implementation from the representation used when you send/receive data via the network. Don't use Python dicts. Use a widely accepted serialization format like JSON, ASN.1 or protocol buffers. Make sure you have a clear idea what you need to send over the network and what the requirements are (latency, throughput, CPU time for encoding/decoding, etc) and choose something that fits them.
Second, use a de facto or de jure standard for communicating over the network. Be it REST, AMQP or anything else - it's impossible to tell which one would be the best fit since your question is too broad. But make sure you're not implementing your own in-house adhoc application layer protocol - you would just make your life and your colleagues life so much harder down the road.
I'd suggest you think a bit more what you want to do, and post more specific questions later on.
Data Handling For Network Transmission