My experience on eliminating twang has been a few things. First, order online from a large vendor with fast turnover (AHS, Midwest, NB, B3, Etc). Their ingerdients are fresh. I, for example, orderly mostly from AHS, and I have never had any twang from their LME or DME. Secondly, I moved to partial mashes. I don't have all the resources to to do all-grain, but I do have the resources to do PMs with about 4-5 lbs of grain. Go to walmart and buy a 2-3 gallon cooler to mash in or use DeathBrewer's technique. Both work just as well with limited space and resources. In conjuction with partial mashing, I add all my extract in with 15 minutes left. None goes in before then and I use the wort from my partial mashes and some top-off water to do the full 60 minute boils. I should note this is using a 2.5-3 gallon boil.
Lastly, and this is a personal opinion and not a fact, but I feel people blame extract twang on tastes that aren't really twang, though no doubt twang exists. The majority, but not all, of brewers who use extract are relatively new to the game, myslef included, and we aren't exactly qualitfied to discern what is twang and what is caused by something else in our process. I've made beer with "twang," and I've made beer with twang. Catch my drift?