So you are talking about marketing. A sales plan is really nothing more than that. Anyone is good at pointing out "ways" you can sell something but the efficient Entreprenur will point out step by step how to do that in the sales plan.
If you have a prototype, begin showing to your potential future clients. Create a detailed description and persona of your typical customer... From the intent of use, frequency, typical hangout spots, number of friends or coworkers, nature of that friendship, nature of business approach, wealth, commute... Anything... Trust me- this helps a lot in curating your first list of leads to approach first. If you get that going those same leads could generate valuable referrals.

Now a days you don't have to spend your lifetime funding manufacturing on whim. Try leveraging social media and crowd sourcing for that as well and that will help you not only validate but also fund your first version and gain that first round of "customers"

not to pitch here, but give me a call if you want to discuss your opportunities here a bit more :)

Best of luck!