Some of it depends on your profile:
Nationality, occupation, age, purpose of visit. Where you will be staying. Frequency/duration of previous visits. And so on.
Most tourists stay on average 2.5 days. So to renew a 30 day SVP I think you need to have clear and convincing reasons why you are so different from the average visitor. And be aware the suggested 3 day side-trip to seek to renew your SVP on re-entry can be a high-risk thing to do [x-ref to the paragraph above]. Basically it's pretty desperate and a bad idea, and against the spirit of the rules.
As an alternative you might be able to apply for:
http://www.ica.gov.sg/services_centre_o ... &secid=178
'Extend your short-term visit pass'.