Bring your voice slightly below your normal speaking range. Higher registers may sound thin or wispy when on air, so you will want to find a comfortable range just below your talking voice. It should not be a dramatic change (just enough for you to notice), since going too far below your normal speaking voice will sound unnatural.
Speak clearly and enunciate your words. It may sound strange when speaking naturally, but over-enunciating your words will help convey your voice more clearly when on the radio.
Extend your vowel sounds longer than normal. A common trait among radio DJs is over-extended vowel sounds (such as the "o" in "morning") used for effect. This gives the cadence of speech a playful character. It also helps to ensure that each word is both audible and measured.
Practice by speaking into a tape recorder. It can be embarrassing when listening back the first time since your voice will not sound the same recorded as when you are speaking, but becoming familiar with the way other people hear your voice will help you to better control it.
Remain calm. A DJ must always sound comfortable in front of the microphone. For this reason, be sure to practice your technique until you are no longer thinking about doing it -- it should be second nature by the time you go on air.