Find steeper hills!
If you have a mountain bike then you will run out of gears on the road as they are often set with a fairly low range for - well mountains and off road stuff. Sort of like a tractor.
If you are on the road then get a bike for the road! You should then find you will not run out of gears until you are doing 40mph!
I run out of gears on my fold up and mountain bike on the road but hardly go into top gear on my racer.