Force Perspective help needed

Use the hills to your advantage. Each crest line will lead to a smaller batch of trees.

Ok, but does the size of the hill(s) matter? This may be a stupid question, but I felt I should ask it. May be I should very the size of the hills?

In order to carry out forced perspective effectively, you will need a box diorama to limit the viewing area. Using Philip's idea above, each ridge line will allow you to use a smaller scale. Think of looking over a series of ridges. Each vista is significantly farther back, hence the details look smaller with out a continuum of visual shrinkage something like this:
v
v
v
v
v
v
Where each underscore represents aridge.

Thanks! I get the idea now. So, each of the hills behind my subject will require smaller vegitation as it goes towards the rear of my dio.

You have the idea.
And don't make the hills the same height difference as you move back, would look unatural.
I also wouldn't do too many. Stick with between 2-4.
The nice thing about a box dio is you can really put these almost right together at the back and still have a lot of room for your main dio.

T
H T
H H
H H T
H H H T main action

Pardon my rough attempt here. This would be looking at your dio from one end with the back of the box to the left. The T stands for trees and the H for hills with the main action (tank, building, artillery, etc) taking place at the right (or front of the box).

Thanks. Will try it out this weekend.

I think it would help using lighter and lighter colours as hills are farther, just as they do in paintings