My barrel saddle slides forward on my mare, I use shims to help fill it in some because it has a gullet that is a bit too big for her but it still slips forward on her shoulders. I use a back cinch and I have a csi pad with a 3/4 inch liner if that helps. Would a non slip pad liner help maybe? I know the ideal thing would be to get another saddle but right now that is not an option. Any suggestions to help in the meantime?

Pics would help. 

I can hardly ever get pictures to load on here but I'll try. 

If you are already shimming and it is still too wide, there's really not much else you can do if the saddle doesn't fit from the get go.

I'd try it without the liner.exit: try a neoprene pad as your base.

I didn't take into consideration my mare is a bit downhill. I did some research and I found some articles about taking a center or back shim and then putting a shoulder shim with it to make a "ramp" ? I guess this helps downhill horses to get the saddle to sit right without bridging? Anyone have tips for fitting or shimming a downhill horse?

Both downhill horses I have rode have benefit from shims over the shoulder. However, gullet is less important to me than bar angle. If your horse is with through the back/loin, your saddle may be pushed forward beyond what a shim can stop.