I have used GW trees, though i think that they are quite expensive and you only get about 4 for $20-30 AUD. And a decent looking forest has around 10 trees in it with plenty of nice undergrowth, so it can be very expensive to buy GW trees.
You
can use real plants, they just need to be sprayed with a sealant like a aerosol glue spray or an enamel so they dont decay and break up.
Or you can make your own from florists' wire, bandaged and plaster. Though it takes a lot of time,patience and skill if you wanted to, plus you need to paint them yourself...
But i think if you have the money, go buy the GW trees, they are good quality and look the best. If you want more advice then i suggest you look at the 'Scenery Spotlight: Trees' below the
Fields of Battle subject on the home page.
Cheers!
--DGlade--