Guess I am weird but I found spirals easier to use and more natural to me than flat blades. Now using a flat blade is like starting over for me.
Dick somethings to try/think about with when using them.
FIRST, relax, breath normal and do not force them.
I concentrate on watching the side of the blade when cutting, i.e., keep the side of the blade at the line.
You can still turn the board. I do not try to only move the board forward and back and side to side, I will turn the the board also. I find I have the most control when I am pushing the wood away from me, so I sort of try to go that direction. I do not turn the board all the time to follow a line but if it is a long cut, I will turn the piece so I am doing mostly a push cut. Hard for me to explain, hope it makes some sense.
Tension is important, if the blade is wondering somewhat, not going exactly as you want, try a little more tension.
Experiment with speed. Personally I usually run at max speed. Others like slower speed. Find what works best for you.
I have the most control (again this is me, may be different for others) when cutting approximately 1/2" thick material. Baltic Birch I always like to stack two 1/4" pieces. My preference is to cut solid wood though but it is more expensive and each piece is one on, so if you are selling or something, may not be feasible.
My absolute favorite blade is the 3/0 New Spiral Flying Dutchman. But be prepared, to buy them by the gross. I break a lot of blades. Now I am trying to cut the bigger areas with a 2/0 New Spiral and using the smaller 3/0 for the small delicate cuts. It is saving me blades, but I still like the 3/0 best.
That is all I can think of right now. Glad to see you give them a try. I really think you will like them for portrait work.