Well, hah...
Anything I could say would not fit on what you are looking for, there are so many ways to make trees and all them are valid, so consider that. If you need references you can search on pixeljoint or on this forum, there are thousands, millions maybe. Billions not.
You can make the foliage leaf by leaf or all in one, and there also are so much ways to pixelate that foliage. I like the leaf-by-leaf option when it's suposed to be a big leaves tree, where you can pixel all one by one without spending too much time.
In opposite way you can efficiently pixel all the folliage playing with shadows.
The 'sane' combination of both techniques usually is the best option.
Back on your tries, they are too plane: they need more detail and shadows. Looking the type of tree you did, I would recomend something like that:
Beware also your pallete, this last example was 22 colors and it's enough. Your trees need to be indexed on a better pallete, they have too many colors. Edit: Ah well sorry it's because it is in jpg, I didn't see that. Use always png or gif.
Oh I forgot about that. This is my favourite pixelart, and it has a big tree:
http://kawaiihannah.deviantart.com/art/Tonari-no-Totoro-350642231Look how the shadows are.