Grades
What is the right grade?
The first thing you have to decide is how steep you want your grades to be. Model trains can handle a lot more than the real thing, so space constraints frequently argue for some fairly stiff grades. However the grades they can handle do vary quite a bit between manufacturers, locomotive, and consist. If you do not already have some way of measuring the pulling power of the locomotives you plan on using, it is wise to first rig up a temporary uphill run. The easiest way to do this is to stick some track to a piece of wood, such as a yardstick, (my local hardware store gives them away, so I use them for all kinds of things), and experiment. Prop the wood up at various angles, and try different combinations of locomotives and rolling stock. You will get a feel for what is acceptable to you in short order.
The prototype generally prefers to keep grades within the 1 to 2 percent range. They would find a 5% grade to be a real problem, although some city industry tracks can go as steep as 8 percent. On a model layout 5% - 6% is very acceptable grade, if used in moderation. On my layout I consider 5% to be the maximum allowable grade, though I have been known to violate my own rules when the design didn't fit into what I wanted to do. I would consider a 4% grade to be a good compromise between efficiency, prototypical accuracy, and ease of design. If you can fit a 4% grade into the space you have available, I believe you will find that it works well and looks very prototypical.
Not just One Grade
One word of caution to bear in mind. If your grade is going to be greater than about 2%, you need to be particularly careful to provide transition grades onto and off the main grade. If you lurch right into a 5% grade off of a flat level track it may be too much for you locomotive to handle. The longer the locomotive the more likely you are to have problems with it (this is why Shay's were so short). It's body may catch on the grade, and/or it may loose electrical connection.
When you are figuring out the length of the run, add some distance at each end of the main grade for the transition grade. This should be at least one, preferably two, locomotives in length, and at half the grade of the main. John Armstrong's "Track Planning" book has a lot of information on this, as well as a table to do the calculations for you.
The Easy Way
I hate to admit it, but I am not a big fan of planning. It's definitely worthwhile, and I strongly recommend that other people do it, but I myself just don't have the patience. I'd rather just work the layout as the mood strikes me. Sometime that works great, sometimes.... , well a little planning can save a lot of rework. However if you also prefer to work this way, here is a quick and easy way to set up your grades.
First off you need some device for measuring the current grade, I myself use a gauge from MicroMark that indicates the current grade based on how much it's tilted. Take a long skinny piece of wood, such as a yardstick, and prop it up into the approximate location where you plan on placing the grade. Place the gauge on top of the wood to take a reading of the grade. Now adjust the length of the run or the elevation of the grade to achieve the result you want. Once you have the beginning and end points of your grade fixed you can replace the yardstick with the roadbed appropriate for your layout.
This is not very precise way to approach things, but it's fun and seems to work out fairly well.
The Precise Way
For the numerically inclined among us there are a few easy formulas to compute grades. First let me define a few terms (there are only three):
Grade% - The grade expressed in percentage ( 3 = 3%)
Rise - The height you need to raise the track to
Run - The length of the run required to reach the desired rise given a specified grade.
The basic formula is Grade% = (Rise/Run) x 100
Of course, it is assumed that all measurements are in the same unit, usually inches.
As an example, suppose that you has a 300" run up to a height of 6". The grade on this would be (6/300) x 100, which equals 2, or 2%. Below are the various permutations of the basic formula, all of which are equivalent. Just choose the formula that solves for the one variable that you don't know.
Grade% = Rise / Run x 100
Rise = Grade% / 100 x Run
Run = 100 / Grade% x Rise
Happy Gradin'