Livecurlylivefree.com has the best explanation of co-washing I've seen anywhere, in terms of the technique and such.
I also tend to get oily grimy roots, and while CG didn't eliminate it, it did seem to make it better (part of this is that I do intense martail arts 3-5 days a week for 2-3 hours a day). The important thing to remember is that there are countless ways to tailor CG to work for you (and that doesn't even count the realm of "modified CG").
I would recommend using a very light conditioner to co-wash and a richer conditioner to use as a traditional rinse-out conditioner. My personal favorite for co-washing is VO5 Vanilla Mint Tea.
If co-washing doesn't work for you (and it doesn't work for everyone), you also have the option of a cleansing conditioner (like Deva No-Poo or Curl Junkie Daily Fix) or a low-poo (sulfate-free shampoo- tons of options). Many people, especially with finer textured hair, will switch between these three options as needed.
3a/f/iii Before the chop- Donated to Locks of Love, January 2013
Modified CG since 11/5/11
CO-WASH: VO5 Vanilla Mint Tea Clarifying, DevaCare No-Poo
CLARIFY: CHS Treatment Shampoo (used prior to PT/DT)
RO: DevaCare One Condition, Regis Olive Oil
LI: Cure Care, As I Am Leave-In
STYLE: Re:Coil, Curl Keeper, Biotera Gel, Deva Ultra Defining Gel, GVP Liquid Sculpting Gel, Curls Rock Amplifier and Strong Hold Mousse
PT/DT: IAGirl's PT, One n' Only Argan Hydrating Mask