The End. The Final Curtain. The Grand Finale. The Finish Line. However you think about it, the ending is one of the most important parts of the song. The most important thing about the ending of a song is that you finish on the root note or the note, which the scale is named after. The easiest way to end a song is just sustaining the last note but there are many other ways to end a song and I have listed some below.
A very commonly used technique which is a gradual decrease in volume until the song can no longer be heard. It sounds as though the music is walking away from you. Fading can be quick or slow and is usually used on guitar solos, intro's used as endings or the chorus.
Dynamics And Tempo Changes
Used in every ending to some degree. Either getting louder or softer or faster or slower. Obvious. Yes. Simple. Yes but it's what happens so I don't care if you say, "Tell us something we don't know". Anyway next point.
The Texture Of The Sound
The number of instruments you can hear playing at the end can increase to the whole band or decrease to just vocals.
Leading up to the ending of a song a key change may be introduced to make the melody sound different to interest the listener but not changing the notes.
There is also a technique where the music suddenly stops and you don't expect it to. It's like the music crashed into a brick wall. I don't know what its called but you get the point.
There is also that huge climatic ending usually used at live concerts where the drummer goes nuts, the guitarists use tremolo picking and strumming, the lead guitarist rips out a monster solo, the bass player wakes up out his deep sleep and would either play tremolo or follow the lead guitar and the singer would scream till his throat is sore and red! A great way to end a song and a favourite of mine. (It usually makes the hair on my back stand on end but in a good way!)
Or like a Pink Floyd album, you dont even need to end the song but just play through to the next song or play sound effects etc, just listen to one of their albums and note how they do it. It is very effective and you often wonder "Which track am I listening to now!!!" I'd also like to add that Pink Floyd are a great band and Dark Side Of The Moon is one of the best albums I've heard (As well as AC/DC's Back In Black, Green Day's American Idiot and System of a Down's Mesmerize etc)
That's all that I can think of for now, I have more than likely missed out some important and obvious ones so if you have any please post them below the article (it is my 1st article so don't be too harsh).