Do feel free to wear long hair even if you are a man. Singapore does not have prohibition against long hair, contrary to media reports in the past.
Do not bring drugs such as cocaine and heroine into the country. Anyone who does so will be prosecuted and the mandatory sentence if found guilty is capital punishment, with no chance for a reduced sentence or pardon. Zero chance. So, don't do it!
Do not bring fresh meat into the country unless you have obtain an official permit to do so. Check with the Singapore embassy in your country about this before your trip.
Do not bring any animals, birds, reptiles and insects into or out of the country unless you are sure that doing so will not violate international conventions governing such actions, because Singapore has signed such international agreements and enforces them.