If you're trying to identify a song simply using a melody, it is first important to consider how much of that melody you actually remember. Every little bit of information helps, and will increase the likelihood that a friend will be able to pinpoint the song for you.  Being a more attentive listener throughout your music-listening habits will make future scenarios like this much easier to grapple with.  Take care to make sure that all of the information you think you remember is actually right. The memory can be a strange thing, and a few botched notes in your melody can lead your helper completely astray. Choosing the right person to help you has a lot to do with what you currently remember of the song itself. If you distinctly remember the song's melody chances are you'll know the genre as well. Many people prefer certain genres over others, and the ones that tend to listen to the genre of the song in question will subsequently have the highest chance of successfully helping you out. Enlisting the help of friends with formal musical training is helpful too, as they are used to identifying music purely on the basis of melody. Find somewhere non-distracting and relatively quiet; that way, there won't be uncontrolled factors hindering the process. Sit down with your friend and recreate the melody for them, whether with your voice or the piano. Take care to accurately reproduce the cadence and rhythm of the melody as well-- melody entails far more than a simple sequence of notes! While you can technically recreate the melody on just about any musical instrument, it's actually recommended you try doing it with your voice. Especially if you're not a trained musician yourself, the human voice offers a lot of potential for expression. In singing the melody, you might be able to get some of the timbre and tone across as well, making your approximation that much more specific. Giving your helper some context as to the tone, rhythm and general style of the song can help them pinpoint a song just as well as any melody. Part of what you're doing is trying to give them an accurate impression inside their head as to the song. That way, they'll have an easier time to pinpointing it. Many melodies can be used by multiple songs, so providing a little bit of context can go a long way towards helping identifying the song in question. Now that you've given them the melody, you and your friend can brainstorm as to what it's about. Encourage them to ask you questions about the song. It is possible you still remember more than the information you've offered them. Give another hum to the melody once the two of you have discussed it a little bit, and see if either of you approach it any differently. While you're talking and humming the melody, you should remain open to the possibility that you'll remember the song yourself, provided it's something you've forgotten as opposed to trying to identify it from scratch. If the first person you ask isn't successful, you should try getting help from other people. In the case of identifying a melody, in most cases it's a simple matter of having it click instantly. What ends up leading to the brainstorming stage for some people may be an instant bid for others. Don't lose hope if you can't get it right the first time; find someone new, and begin the process again.

Summary:
Consider what you know of the song. Find a friend who might know the song. Play or sing the melody. Describe the other parts of the music you remember. Brainstorm. Ask others.