Trance has to be the genre of electronic dance music in which I have the most understanding. Allow me to share my knowledge of the basic elements of trance.
4/4 beat, as stated above. Almost always in a typical [kick] [hihat] [kick+snare] [hihat] loop. Can be anywhere between 130 and 150bpm.
Trance usually combines ambient elements and sounds throughout most of the song, while still being energetic.
The basic structure of most trance songs is the it builds and builds and builds throughout the song, with some breaks and suspensions in between. Trance might usually start with the beat and nothing else, but that is quite boring in my opinion. I like to begin trance with the beat (kick drum) and the bass, and add on from there.
It goes like this: The beginning of the song briefly introduces you to the song, starting with very little - usually just the beat and bass. The song must build and get bigger from here. Then it expands with some small melodies and other instruments/sounds of your choosing. The chord progression is also very minimal at this stage of the trance song, and by that I mean it shouldn't include all of the chords of the main chord sequence that will appear at the main section of the song. Usually the chord sequence will only include one or two chords at this stage. From here, the music might build more and more until it "releases."
The release is when all of the percussion disappears and the song becomes more ambient and smooth. This might also be called the mid-song "breakdown." Usually, this is the part where the song is calm and one might find it rather easy to fall asleep to. After a few bars in this part of the song, the main chord progression and the main melody are introduced to the listener, still with no beat. The main chord progression and melody continue to play for a few bars, the song still pretty calm. Then, the song might build up some more until the drop.
Yes, there are drops in trance music too, but in trance, this is simply when the beat and the main bass reappear in the song. Except now, they are joined by the main chord progression and the main melody, as they were introduced earlier in the "release" section. Then everything plays together in perfect harmony and continues to build a little bit more. Maybe you might like to add some more synths playing in the background, or some other instruments to join in on the fun. This is basically the most fun and exciting part of the song.
From there, the trance song builds to a climax, before it begins its decent back to planet earth, by slowly reducing itself from all buildup it has done. The chord progression switches back to the minimal chord progression that was played at the very beginning of the song. Instruments and sounds will slowly back out and disappear from the song over the course of this part, usually one by one, until all that's left is the beat and the bass again.
The song might end however you like, but trance usually ends with just the beat and the bass playing.
The whole point of popular trance is that it BUILDS, from almost nothing, into an exciting piece of music throughout the song. The aspect of increasing tension and musical elements in popular trance is pretty essential, like hardcore wubs are essential to brostep and popular electro house.
Now, you don't have to do everything exactly like I say. You don't even have to follow what everyone else does when they make trance music because the purpose of music is to express yourself or make something that YOU like to make and listen to. That being said, your trance song can follow any structure that you want as long as you have that 4/4 beat that has kick drum on every downbeat and a regular open hi-hat placed on the upbeat or every 1/8th division of the bar - if you want it to be called a "trance" song.
Armin van Buuren, Ferry Corsten, and Tiesto are all very popular trance artists. Try looking up some of their songs for possible ideas.
You might like to check out these examples:
http://www.youtube.com/watch?v=0nODdkoQ3pMhttp://www.youtube.com/watch?v=RTGslPtZNh8