For monofinning good back and schoulder flexibility is very important.
There are exercises for it, and many of them can also be found in yoga books.
I also recomment you to buy a big 85cm inflateble rubber ball, its a very good tool to gently stretch your back.
Make sure you do every stretch gently.
Stetch frequently, its best to do every day a mild but good stretch.
Doing only once or twice a week a hard stretch session is not good nor healthy.
Becomming lean is going to take time, but wel worth it. As you'll become more flexible your abilities to produce a good monofinstroke grow. I find that with every cm of reach I can do a much better stoke and everything I've learn is finally be able to fit and fall in place.
If one happens to be very stiff by nature, and one isn't willing to invest in flexibillity one is, by far, better of with a pair of good carbon stereo fins.
Tom Sietas, martin Stepanec, Umberto Pelizzari, stefane Misfud have shown us that stereo finning is not infereor when it comes to freediving.
To me the monofin feels very natural, that's what I like about it.
Good luck with discovering your way with the monofin!
Kars.
Nathan Watts