I'm not Arabul but I'd like to throw in my opinion. Like Arambul said thieving and agility. Add slayer, hunter, mining, woodcutting and fishing, One doesn't have to spend a lot of money on this skills. It's basically an investment of time. Lots of time in order to level these accts. There is no way to significantly speed up the process. "Buying" a skill means spending a lot of money, millions and millions of gp, in order to level the skills. The normal way to level the secondary skills is to do the primary skills and then use the items to level the secondary skills.