First let me say I think Capability Brown would have loved Second Life. He after all would divert rivers, create lakes - do whatever he wanted to tame the landscape in a way which looked natural not contrived, but exquisitely artistic. Seems contradictory I know, to tame the landscape in order to look natural - but it worked for him. :)
The rest of this post will, I'm afraid, betray a certain degree of arrogance. I'm going to assume as I write this that I have learned enough to actually give advice - and that somebody will actually think reading it is a good idea.
I've been planting the forest on Aintree Common and realized as I did how much work it is to achieve something that looks even quasi natural. So for those of you who want to create a forest, here are a couple of tips.
Plant the forest in layers. You're going to want 5 or 6 different trees to create something with some texture and interest. Start with one and plant it "randomly" throughout the forest area. Random isn't easy - it's time consuming and tedious. You have to think about the placement of each tree and work hard to make it look like the placement is accidental.
Each of your 5 or 6 trees will be planted in a separate "layer". Choose trees which give you some variation in height, shape and colour.
I have some trees in my inventory that I don't think are good enough to sell to people - but they work to give depth to plantings like this.
You aren't going to plant the same number of each tree. The first layer will be the foundation layer - the rest will be more scattered and seemingly arbitrary.
You'll know you're finished planting (which this isn't) when you stop asking yourself "What will I put here?" and start asking yourself "Where will I put this?"
Once you've got the planting done you still have to tweak. Walk the paths you expect your "audience" to walk. The journey is more important than the destination in this case, you have to give them interest for the entire trip. Otherwise they'll just get bored and fly there. :)
OK, I'll stop pontificating now. :)