I dont know how old your bike is but with older bikes you could not cross chain. That is big gear in front and biggest gear in back and same with the smallest gear. My triple 80's era bike is like that so I use the largest 3 or 4 gears in the rear with the smallest front gear, all middle gears in the rear with the middle gear in front, and only the smallest rear gears with the biggest front gear as conditions dictate. Also my bikes have shifters located on the downtube so I don't shift near as much as my ridin partners with the modern shifters. Sometimes I think riders with the convenient modern shifters shift entirely too much. I find often that it is more efficient to power over hills rather than continually having to let go of the handlebars with one hand to shift while climbing. This alos makes me more likely to shift to a low gear pryor to goin up a steep hill.
Good luck,