I second the coconut oil. Jojoba oil is also supposed to be very similar to sebum (natural scalp oil). Actually, the night before wash day, I've been oiling my hair from the ears down fairly generously with a mixture of coconut & jojoba oils and my hair really likes that. (I've been using a sample of really yummy virgin coconut oil I ordered from
www.coconutoil-online.com -- it was $1.50, and if I only use it for my hair it should last for months!)
Anyway, for everyday, I use the tiniest speck of coconut oil (just enough to make my palms very slightly glossy) and smooth that on my last 3" of hair first, then smooth again from the ears down. I do it every couple days.
By changing how much baking soda you use, do you mean you've changed the consistency of the mixture? I had terrible dryness when I used a creamy mixture, but I'm really happy now with a watery one. I don't leave it on, either, I just make sure it's worked in and then rinse right away.
have you tried adding honey? or an egg yolk or some plain unsweetened yogurt? I don't remember if you use anything besides bs and water.
Have you experimented with using a larger amount of vinegar?
Also, does your hair usually get frizzier this time of year? With people in most areas starting to turn on their inside heat, the air's dryer & hair tends not to like that.
Good luck!