There used to be a golden rule of offering half the asking price, but that rule has grown unreliable over the years, as the asking price could be treble the real value or just a fraction above. If you want to obtain a bargain, be prepared to invest some time and have a rough idea what the item is worth. When I return to Egypt, I always relax my haggling rules because shopkeepers know instinctively that I live abroad and raise the price. Ideally, you should take an Egyptian friend to buy for you, but since that is not always feasible, do some homework. Shopkeepers will try to persuade you that they offer you the best value, but will not be offended when you say you want to look around and would come back.
Timing is also important.
You stand a better chance for getting a bargain late at night than earlier in the day. When buying gold haggle about the price per gram not the price per item. For other valuables, you can always say a friend bought a similar item, for less. Negotiate only with those who can make an instant decision, not with counter assistants. If you are not sure, ask to see the manager or owner.