flax to bowstring used to be a great way as you can earn quite a bit. now since the price has dropped, i guess using it for fletching rather than selling may be a better option. do it at lumbridge, its fast and you can get some good deals there.
laws give more xp per rune than nats. however if you're using the abyss, you'll be running naked, and you can only use the agility and thieving paths. which means that if you land on the western side, you gotta run all the way to the other side to get into the centre.
wear it when you're training skills like cooking, firemaking, wc.. those that you spend lots of time standing at one spot. although it works better if you move, i think a spottier cape would be better then.