What phone? If it's an iPhone, you can use low power mode. Android also has a battery saver mode, but I think it disables GPS. In either case, you can do a lot of the things that these modes do by themselves--turn off bluetooth, turn down screen brightness, turn off anything that is fetching data. Of course, if you have the screen running with your GPS app on it the whole time, that's going to dominate your battery usage, so the best thing you can do aside from turning down brightness is to try to turn the screen off if you don't need to look at the app for long stretches.