Hey there, I don't know if you're looking for pure hand-drawn advices or hand drawn used in softwares : in PS (or GIMP) you have several options to hand-draw forests. Those I used it either draws indivual ones and turned them into brushes in PS then build forests (examples on some of my maps here and here or do the same thing and use trees as pattern with clone tool ( another examples here and here and a small tut on how doing it here . Also you can directly draw the whole forests with a similar way you started to do (see another example here. Torstan has some useful tips on his thread here for hand-drawn forests too
Hope it helps