1. You won't be much use if you can't listen. When you go abroad to volunteer, you are joining a project that has been working on the ground for a long time. These people have committed themselves to solving a real problem existing in their community. Listen to them. Hear their stories. Learn about their cultures. Don't assume you understand the problem. Look at all factors of the situation, and most importantly- do so without judgement.
2. Some volunteers are unwilling or simply cannot to adapt to a change of pace. One of the biggest shocks for volunteers when volunteering abroad is the change of the pace in work. Many countries operate on a different clock. My appointment showing up 20 minutes late to an important meeting is not unusual in Quito, despite the fact, that I myself, show up at least five minutes early to an appointment when home in Chicago. Don't get angry with something that is going to get done tomorrow even though it could have easily have gotten done an hour ago. Communication, work pace, and access to supplies can differ greatly from country to country and even region to region. Work with the grain rather than against it. Don't expect everyone to conform to you. Be ready to adapt.
3. Don't come thinking you have a million-dollar idea. Sharing your skills and expertise is essential, but you can only understand how to make a sustainable impact once you understand how the project works within the community. Some ideas you think are great, might actually not be. A friend of mine told me that during a volunteer trip to Argentina, he suggested helping fund the project by collecting recyclable materials. They informed him, they had already considered that option, but decided against it, because the poorest of the community rely on collecting recyclables in order to earn extra money to simply get by. Collecting recyclables to fund their own projects would take away many of the most vulnerable people's livelihoods. These projects are working to positively effect change without hindering someone else.
4. Don't be scared to step outside of your volunteering role. Above I describe into an unfamiliar place with a million ideas to implement. Now I'm telling you coming in and solely sticking to your volunteer duties when you know you can do more is a shame. If you see a chance to use your skills to help out in something which the organization is having trouble with, and you can make a difference in the time you are there, share your idea with those in charge. You might be surprised how willing the organization is wanting you to fulfill that need. If you're an electrician, can you fix the faulty wiring? Perhaps you can use your English skills to update their webpage. Teaching a new skill to the local women, like a new way of making jewelry, can help boost their collective income. Just remember, make sure it is something the project can handle and maintain when you are not around.
Understanding what the project needs, what its working towards, and what will be useful is essential. Take the time to listen, adapt, learn, and work toward a common goal.