Wear a condom. It sounds too simple to be true, but it works for a lot of men. Condoms reduce stimulation for most men, which should prolong the time before ejaculation.[1]
Steer clear of condoms that are marketed as extra thin. You can always upgrade to a brand that allows more stimulation later.
Some condoms are coated with a slight numbing gel on the inside. This can help you put off ejaculation for longer without causing numbness to your partner. (Just make sure you know which side is where when you put it on.)