It all depends on how you sell yourself to them. If you run into a problem of being "free" though and you really don't want to charge, you could perhaps call it an "internship". I think if it's called an internship, it's kinda implied that you want a recommendation afterward.
Personally, I would not suggest working for free. You will get trampled all over, and only attract the deadbeat customers who will always be hassling you for more. It is not worth it my friend, this is your life!
If you are working for free, work for yourself for free. Get out there and sell yourself as a consultant for say $997/mo. If you are confident you can increase a businesses sales, you should have no problem getting that amount. Action, confidence, and persistence will get you much farther than working for some deadbeat who only takes on free labor.
Hope this helps!
If you work for free, people will be suspect that that's exactly what you're worth. To be successful with increasing a business's revenues, the owners have to be as invested as you are in your recommendations to market in a different way, go after a diffferent niche, hire and fire, motivate the staff, invest in whatever is needed to systemitize their operations. They won't invest in these necessary tools, staff or endeavors unless they believe in you. And they won't believe in you unless you engage them to have some skin in the game up front.
Here is something else I've seen businesses do successfully. They declare that they do one probono project per year for a non-profit (put it on your website). They find one or more non-profits who want this work from you (you can make it a contest or at least an application process). Let them know you will expect a testimonial, a referral and a case study as well as some resources to follow your suggestions. Then you do the probono work. Most likely there are people on their board of directors who might take a liking to your work and offer you other opportunities. It's a good way to get started. I hope you are successful.