1. Get a local SIM card; circulate the number to your most frequent callers.
2. Keep off data roaming. The bills will make you weep
3. If you can, make extensive use of VOIP services eg Skype
4. If you must roam, try to "latch" onto a local provider who may have preferential roaming rates with your home provider eg Vodacom affiliates work better with Safcom, Airtel sister groups blend better with Airtel Kenya, the Orange group favours their own brands etc..
5. As a rule of thumb.... pre-paid roaming (if allowed) has lesser disputes than post-paid roaming.

Aside : Previously, Zain group had that God-sent pre-paid roaming service (One network) that was silently discontinued by Airtel. Oh the handy acrobatics that went with it!