There are really four things you need to do with the wheel and truing is only one of them. The rim also has to be centered (called dishing), the spoke tension as even as you can (imperfections in the rim can make it impossible to get perfectly even tension, but you should be able to get close, if not time to replace the rim). The fourth thing is it has to be round. This is why when you true you tighten one spoke you loosen the spokes next to it - so the wheel stays round. However, in your case, when the wheel went out of true it probably also went out of round because the going out of true was cause by spokes loosening (and they didn't all loosen the same amount). This means that your loosening adjacent spokes when tightening a spoke is probably just perpetuating the out of round shape! What you really should do with the back wheel, which was way out of true, is to loosen all the spokes and tighten them back up evenly. Do this slowly and check the true, round and dish and tension as you go along and don't let any of these get too far out. Checking the round and dish can be tough without a truing stand but it can be done. Dish and be checked by turning the wheel around and putting it back in the frame - it should be in the same place it was before it was turned if it is in dish. To check round you need to remove the tire and rig up something to touch the outer edge of the rim. Tighten the spokes where it hits and loosen where it gets far away. It is really not that hard, just takes patience. The key is to control all four factors so that when you are adjusting one you don't knock one of the other factors out by too much.