My advice is to a) get a copy of Anyrail
and b) have something temporary set up or semi-permanent that you can play with when you are bored of doing track planning logistics
Also put together a list of what you would LIKE to have in your layout. For example, when I do a HO layout I would like:
shadow station for 3-4 trains
multi-train control, automation of passenger service
mountainous terrain allowing for good photo opportunities
station large enough to hold 3-4 trains
various industries like sawmill, freight station
However it took me a couple years of looking at various plans, other layouts and ideas before I really knew what I wanted.
Here is a plan I came up with that was based on a plan from Marklin Magzine. This is fairly comphrensive and would cost an arm and a leg to complete, so this won't be happening anytime soon. BUT I had alot of fun just doing the plan and working on all the problems with it.