Borrow one of our clinometers or buy a phone app.
Know the slope you're working with (existing trail, potential trail).
Flag ways/routes that fall into the general rules for slope and climbing 1/2 of it. If that or routes don't work you flag to places where you can make turns.
Use a radius greater than 6' before you plan turns and/or think of where a turning platform might be because there's a tendency for them to get smaller when they get built.
When you make your turns be aware of the slope you're climbing in the big picture (your point a to b climb) and any inslope or outslope to address water.
Editor's note: Make radius larger than you think for type of turns that produce what's known as sh_t eating grin.
If you need to make turning platforms (aka climbing turns) and/or crib walls don't start the project until you have the labor and materials lined up.
This formula generally solves all sorts of problems such as happy riders, minimum maintenance.
IMBA no longer has as much stuff on their www site in order to sell their books but the federal govt and American Trails still have a lot of public info. I will let people look at my books but no longer lend them. I'm at $120 spent on copies of one that never seems to get returned.
http://www.imba.com/catalog/book-trail-solutions