How to potty train my hamster?

First of all to make things clear, its only possible to train your hamster to pee in the toilet. You CANNOT train them to poop in the toilet. Because hamsters will eat their own poop sometimes. Yes you may think its disgusting but there are two types of hamster poop. One is the excrement which contain nothing in it just poop & the other kind that they usually eat are poop that contain nutrients that was not fully digested. Hamster digestive system is not the same as human thats why they will need to eat their poop back because it contains some vitamins/nutrients are not absorbed at the first time.
1) Purchase a litter box designed for hamsters or use a small plastic box that the hamster can jump into. Specially designed litter boxes for hamsters have one corner higher than the others to prevent hamster urine from spraying outside of the cage. Most boxes are designed to be attached to the corner of a cage. A few boxes have covers and entrances.

2) Watch your hamster at night and observe which corner of the cage it normally go to pee. Place the litter box in that corner.

3) Fill the litter box one-third full with a specially made litter for hamsters. This litter can be found at pet stores.

4) Pick up hamster urine-soaked bedding and place it in the litter box. If u found fresh hamster urine try to absorb it with tissue paper & put that tissue paper into the hamster toilet. Just put the soaked/wet part in & not the whole tissue paper.

5) Keep adding fresh urine-soaked bedding until the hamster begins to use the toilet. Relax the process usually takes time, it dosent happen overnight.

6) Keep the toilet clean and filled with fresh litter once a week once the hamster is trained on its use.

Reference: http://www.ehow.com/how_2074136_potty-train-hamster.html