Do be aware that the two entities have their own separate postal services, so stamps bought in the Federation cannot be used in the RS and vice versa.
There are three mobile phone networks in Bosnia and Herzegovina: HT ERONET (Mostar), GSMBiH (Sarajevo) and m:tel (Republika Srpska, Banja Luka). You can buy a prepaid SIM card from any network at any kiosk for BAM10 or less.
Respect the religious differences of the people in the region and their effort to move past the Yugoslav civil war. It is important to be careful in areas where there is still tension and to ensure that one does not offend a particular group due to indifference or sheer ignorance.
Similarly, respect the environment. A lot of the country has been saved from pollution and it is important to be careful of one's influences. Moreover, it is equally important to be careful as the rivers tend to be fierce, the mountains and valleys often unguarded and the footing unsure. Always have a tour guide with you or consult a local for advice on the natural dangers and land mines.