There is a rare tree field fenced in that contains teak and mahogany trees. To get in, you will need 100 sticks. Bring dragon hatchet and several urns for woodcutting.
These sticks are dropped by hobgoblins near the fairy ring in Karamja near Shilo.
The mahogany and teaks are just north of Tai Bwo Wannai. The point of this is to chop as many of both kinds of trees as you can to find special teaks and special mahogany logs. Two of these logs will get you a an inventory full of teak/mahogany cut into planks for FREE! Talk to the guy at sawmill east of Varrock. Do not use the right-click option to cut into planks or the free plank method won't work.
You will cut and add to bonfire all the normal logs you cut. The object is to get as many of the special logs as you can. Once you have enough, then you can start saving your mahogany and teak logs.
You will level up both woodcutting and firemaking skills and save lots of money leveling up the construction skill