I assume you have a 7-speed or more and that the differences between gears is not dramatic. If so, then I think your issue is in cadence. Find a cadence that is quick and comfortable to you and shift frequently to maintain that cadence. When approaching a hill, shift a little before you need to. Finally, be cognizant of which gear you're in so that you're using your front and rear gears in close harmony. I have a 9-speed with a triple front and my front-rear shifting pattern is this: 3-9; 3-8; 3-7; 2-7; 2-6; 2-5; 2-4; 2-3; 1-3; 1-2; 1-1. Shifting into gears outside this pattern is extraneous and sometimes counter-productive.