Trekking in Bali is a must try experience.  Below are some tips from our own experience trekking Mount Batur.  Feel free to jump in and add any advice to the below :p:

•  wear appropriate shoes; the last part of the trek (towards the top of the volcano) has a path made only from volcanic sand so you need good shoes (Gabi trekked up to the top in New Balance while I had some slippery Tommy Hilfiger shoes so not good for trekking);
• have a cap or something to cover your head and a long sleeved blouse; the wind is blowing stronger on the mountain and the morning is pretty cold taking into account also the fact that the trek will make you sweat;
• the trek has medium difficulty if you have experience with trekking mountains; for the first timers (like I was) it’s pretty much difficult;
• trekking sticks are not that useful since the path is sandy and with volcanic stones and it’s hard to find a place for support;
• pay attention to the stealing monkeys;
• on top of the mountain eggs boiled in the volcano and friend banana sandwiches are served included in the price of the trek; you can also get hot tea or coffee for small price;
• you can spend time inside the refugee of the guides if it’s too cold outside and you want to take a rest;
• buy trekking tours (as other tours around Bali) from the agencies in the city; in Ubud we found the trekking tour for 25 USD per person while or travel agency was selling the same trek for 87 USD and our guide for 65 USD.