Walkie-Talkie Range Boosting

Is there any way to boost the range of a Motorola Talkabout t9500. This walkie-talkie claims to already have a range of "35 km", but I checked it out and that's only if you are transmitting from an area of very high elevation (mountain) to an area at about sea level. The range on flat gorund is something like 5-6 km.

I was wondering if there was some way to boost the range of the radio like adding a longer antenna or an amplifier or something like that. I'm pretty new to the whole radio racket so please don't go into too much detail.

Thank's a lot guys!