I recently came across an excellent post from someone struggling with that same question. I hope it helps you: http://blog.cloudfour.com/responsive-design-for-apps-part-1/

Although I am currently writing a book later out this year, my "day job" is running a creative agency where we've created several responsive websites, including http://newportdunes.com and http://searchoptimizers.com I find that sketching it out first allows for the constraints of the design to focus on what is most important. My favorite tool for this is UI Stencils responsive sketch pad, found here: http://www.uistencils.com/products/responsive-sketch-pad

I know you are specifically asking about an app and not a website, but I think it can still be a useful tool in the "fingers first" thought process and decision process. In Jason's post, he recommends Appcooker.com as a ipad based tool.

I think you'll find that Mobile or fingers first is a fantastic mindset with little downside as it forces you to get clear on the context of your content, size targets appropriately, and then the desktop design does not suffer for it. We sometimes go the other way, too, but we understand what "fluff" we give up when moving to the mobile side. Google Luke Wroblewski for more excellent info on his Mobile First way of thinking.

Good luck and let us know when your prototype is up!

What a great question! You are on the right track. First, lets define Avatar as "An icon or figure representing a particular person." In business application, an avatar is a representation of your ideal client/customer. A business may have multiple avatars representing several target demographics. Some prefer to distill this down to one person as an “avatar.” I recently interviewed super entreprenuer John Lee Dumas of EOfire.com and he has a very clear Avatar he calls "Jimmy" that he uses for business decisions you may want to check out.

I hope that you have already gone through the exercise of your UVP, or Unique Value Proposition. If not, may I suggest the worksheet on this page, first: http://bit.ly/1kYTLbf Ok, so once you've clarified the why choose me, then start working on your Avatar, that's the who, or as we marketers like to call them: personas that represent segments from your target market.

You have to start somewhere, right? So do. Are you going to first focus locally in your zip code and surrounding area? That can be one piece in helping you visualize your Avatar's lifestyle. Target marketing has to do with breaking your potential global audience into segments, specifically only the potential buyers of our product, service, or cause. As much as we might be tempted, we can’t be all things to all people. We have to commit and put some stakes in the ground. Are you ready to write your personal ad? Who are you looking for?

Some of the most basic questions you should ask in forming an idea of Customer Segmentation has to do with what these people want, need, think, and feel. No time is wasted from this exercise because it will ultimately lead you to where and with who they hang out (their tribe.)

Why is that our desired end point? Because that virtual or real (coffee shop, tradeshow, website, search engine, twitter feed, health fair, street location, podcast, meetup, traditional media) is where I should be hanging out with my product, service or cause and shouting my UVP from the rooftops as part of the conversation! Taking this thought experiment all the way through will also assist you in the critical understanding of whether the segment is large enough for you to be successful.

I like using the Personas app (available on iPad) to put forth a visual representation of my potential target markets, but a white board works too! Literally put in a photo representation of your Avatar with a Name, and start brainstorming out: Think and Feel? See? Hear? Say and Do? Hangs out with/where?

For you, offering your B2B services, maybe explore linked in to find where your peeps hang out and get a clearer idea of, let's call her, Samantha. Samantha is a small business owner of a growing service business whose revenues just got in the black. She has two employees and she's looking to hire another. She is struggling with getting her website up and mobile friendly and feels like she needs to be more effective in communicating what she does. She is overworked, in her mid 40s and recently divorced with 2 boys ages 10, 12. She has little time for fluff and needs guidance in creating a system that will help run her business. Can you help Samantha? :-)

If you'd like to get more tips that sound like this, I'd be tickled if you let me know if Sell Local. Think Global. speaks to you. It's my first book and I'm feeling very vulnerable putting myself out there! eep! Available now on Amazon: http://bit.ly/olgasbook