Thieving is a very useful Members-Only skill that can be used to make quick cash as you get higher in levels. To make big cash you will need at least 50 Thieving, so if you aren't a high enough level, follow these steps to increase your level:

>From level 1-10, pickpocket Men in Edgeville 145 times.
>From level 10-30, steal cakes from the Baker's Stall in Ardougne 764 times.
>From level 30-50, pickpocket Al Kharid Warriors 3,384 times.

Now that you're level 50 thieving, you should be able to easily pickpocket guards to get some good amounts of gp.

>From level 50-70, pickpocket guards 13,684 times to steal 410,500gp.
>From 70-80, pickpocket watchmen to thieve another 364,520gp and 9,113 bread.

At level 80, it should be easier to pickpocket Paladins to make 100,000gp an hour. Just be sure to bring lots of food with you