Be agressive...polite, but agressive and don't be afraid to walk away. This is a way of life for these people so you should respect that. Don't insult by giving a ridiculously small offer.
Start your bidding at about the 30% mark and don't be the first to shift price, make them come down before you go up. Don't be so eager to go up, make them work for their money.
You should be looking to pay somewhere around the 60% mark as your finishing price. Don't be afraid to accept a drink from the vendor, this is also cultural and a social event.
If there are two of you going you should go for the good cop/bad cop approach.
For example, why my wife and I barter, we have already arranged beforehand what we like and what we want to buy. When we approach the vendor, one of us will pretend not to like or want the item and then the vendor is working against the person pretending not to like in order to get the person who is doing the bartering to buy their product. I have found this to be particularly effective.
Remember one thing, don't feel guilty about bringing the price down and any sob stories you may here from the vendor. If they are not going to make a profit from the sale, they won't sell.
Also if you later find you could have bought the item at a cheaper price, ask yourself this - Are you happy with the price you paid, if yes, then you have not been ripped off.
I have experience in bartering in places all over the world, Vietnam, Thailand, Turkey Mexico amongst other places... Get the hang of it an it is actually a lot of fun. I have many a time bought a useless souvenir item I did not particularly want just for the sake of bartering.