The first thing I do when I was in Singapore is to get myself a prepaid SIM card, roaming is just too costly for me.
There are 3 mobile operator in Singapore, Singtel, MobileOne and StarHub. I just simply get Singtel, the service center is the first to appear in front of me. Later I found out that Singtel is a member of Bridget Alliance. This is the first time I know about Bridge Alliance.
What is Bridge Alliance? Bridget Alliance is a alliance form by few mobile operators in Asia Pacific region. Since both Singtel and Maxis are members, I can top up Singtel with Maxis.
My Singtel prepaid card call Hi!Card. How to top up Hi!Card? Here is a FAQ, you need to purchase a Singtel top up card, or you go to a Singtel service center for top up. If you are outside of Singapore how do you purchase the Singtel top up card? You can ask your Singapore friend do it for you, or there is an easier way if you are in Kuala Lumpur. You just go to the Maxis HQ just beside KLCC, you can do the top up there, because Maxis and Singtel are members of Mobile Bridge Alliance.
The full list of the 11 members can be found here.