Know when to ask for a song
There's a good time and a bad time to request a song. Knowing when to ask for a song is key. A DJ's main concern is to keep the party going based on what type of music works for what crowd. If a DJ feels that electro is working for the crowd, don't ask for salsa music. If a DJ is rocking a dope hip hop set, don't ask for house. It's common sense; ask for songs that will add to his set and not disrupt the flow of the crowd. You're not the only person the DJ is trying to entertain.
Know what song to ask
When you go up to the DJ, always have the song title and artist name handy. That will easily get your song considered and possibly played. Here are some "no-no's" when requesting a song.
1. Please know what genre of music the DJ is playing. Don't ask for "Hip Hop" when he is playing Hip hop.
2. Do not go to the DJ and ask "what else do you have?".
3. Do not go to the DJ and ask "can you play the song where they go '1..2..3...'". Seriously, someone asked me this.
4. Do not go to the DJ and sing the song that has the words "1..2..3..". I may know it, but I won't care at that point. I have a mix to pay attention to.
Know how to ask
Don't be a jack ass and expect the DJ to pay attention to you right away when you come up. Frankly, transitioning to the next song is more important to a DJ than your company at that moment. Wait patiently and grab the DJ's attention now and then; eye contact is always good, unless of course the DJ has his back turned to you.
Once you have the DJ's attention, politely ask for the song and do not keep coming back if the DJ hasn't played it yet. Once or twice, fine, but any more than that, it's just annoying.
These are just some personal pointers based on how people have come up to me in the past. It may or may not work. Bottom line is, use common sense and be polite and don't let it ruin your night if your song doesn't get played. You're not at the event for one song. Good luck!