I just sent out our application today for the K-1 visa.

Any advice to what we should do in order to prepare for the next step while we wait for the notifications? (e.g. additional paper work, etc.)

Thanks

read the guides on here. start preparing for the interview. keep gathering evidence and as others suggested to me...look up the info on the consulates website of where your fiancee will have the interview to see what requierments they have and their process..

Many congratulations to you!!!!
Check out the K1 flowchart here to get an idea of what is coming up next in the process.
Best of luck to you!!!