If you have scalp buildup, try combining 1 tablespoon of brown sugar with the conditioner you're going to use to co-wash, then give yourself an extra good scalp massage when you do it. Some people need to do sugar scrubs on a fairly regular basis. I found that the first one I did helped tremendously. I repeated it about a week later, then haven't felt the need. I think I got rid of old buildup from products with cones, but I probably also learned how to do better scalp massaging in the process.
I've had really good luck with Nature's Gate conditioner. I get it from VitaCost
when I'm putting in an order for Kiss My Face Upper Management, as their price for that is almost $3 less than usual (and I always order a few other things, like jojoba or argan oil – CSI 100% Pure Moroccan Argan Oil is on BOGO right now). 12 oz of the NG conditioner I like best (Lavender and Aloe) is $5.88, but some of their conditioners (such as the aloe without lavender) run $4.89 for 18 oz. (Theoretically, my local supermarket carries Nature's Gate in their organics section, but they've been remodelling for forever.) Shipping is free over $49 at VitaCost, but they sometimes do free shipping on orders over $25. If you're interested, PM me: new customers and whoever refers them both get $10 off an order.