Give us as many details of your car, it's condition, mods, driving style, instrumentation and route and we can surely help you out
the more info we get, the easier it will be for us to nail down how to help.
35 should not be too difficult, unless the transmission is slipping. I was averaging that before coming here in my sporty 98 Escort ZX2.
My guess is that you are not controlling the throttle like it needs when cruising. Back in the winter, I did a quick test with this. At 50 mph, I could get 37-44 instant mpg by manipulating the throttle, but going the exact same speed. Those extra 7 mpg added up quickly!