One thing you could do is create, or download, a seamless tile pattern for your forest, I'm assuming you're using Photoshop or something very similar. Then create a new layer and fill it with the pattern. Put a layer mask over the top of it 'and paint' in the bulk of your forest, then just put some trunks where they would be visible and some individual trees and copses to make it look less pattern like. Butch Curry did an excellent series on YouTube; Fantasy cartography with Adobe Photoshop, if I recall correctly. Also, take at Jonathon Roberts' (aka Torstan here) fantastic Maps site. he has plenty of tips & tricks.