This is a very active volcano. Always be aware of the current assessment of eruptive activity. Sources worth keeping an eye on include local media and The Smithsonian Institute's weekly Volcanic Activity Report (http://www.volcano.si.edu...). Do not always take the word of a trek organiser as gospel on safety issues.
Rinjani is high enough that altitude sickness is a concern. Be aware of the symptoms and treatment.
It is also very cold up on the summit 4 to 5°C, so warm, windproof clothes are a must as are appropriate hiking boots and a head lamp. The sun can be very strong during the day and severe sunburn is a risk.
Make sure that your guide is carrying a radio with him and that it is fully charged and working. A test call to the trek centre is advisable. The trek centre has a satellite telephone and it can be used by visitors - ask to see the chart of charges.
fees/permits
Entry to the park costs Rp 150,000 per person. This fee is split up as follows: 13% to the national park authority, 62% to the Rinjani ecotourism trekking program and 25% for trek route maintenance etc.