The trees are Fine-Leaf Foliage from Woodland Scenics.
It's quite expensive, but my layout is so small so I thought that I could afford it.
Hers what I do, I sort out the peices that look like trees from the foliage and then I take small peices of a bbq-stick. Then I take the tree and the stick and tie it together with some metal wire. After that I drill a hole in the layout and plant the tree with some ordinary white glue. Afterwards you would like to paint the bbq-stick so you dont see the bright wood. I tried painting the bbq-stick before, but the paint only flows of and gets quite messy so I prefer doing it when the tree is planted.
Here are some pics of my trees, I will take better and sharper photographs for you guys as soon as I have something to really show.