When setting up the backstay, you are actually setting the headstay at the same time. As you tighten the backstay, you are simply pulling the mast head aft. This counters the effect of the headstay pulling the mast head forward.
As you tighten the backstay, you want to go forward and check the headstay tension. If you tighten down the backstay too much, it can add un-necessary stress to the headstay and all of the fittings involved.
Tensioning the backstay splits into two categories: Fixed and Adjustable
Fixed backstays must be set up as tight as possible, as there is no way to tension them more while underway.
Adjustable backstays are wonderful because they allow you to fine tune the rigging while you are sailing to achieve the most out of the wind you are in.
There are two types of backstay adjusters, the very expensive hydraulic systems and the very inexpensive pulley type that work well with synthetic rigging. I'm going to discuss the pulley type here.
When you have a split backstay, you simply set up a grommet set up over low friction rings with a line to raise the system (release tension) and a line to lower the system (tighten the system).
As you lower the system, the grommet pinches the two backstays together which then tightens the backstay and also tightens the headstay.
With steel rigging, you will need a large purchase system due to the frictions involved, but with synthetics, friction is practically removed and you don't need the mechanical advantage of a pulley system.
Whichever system you decide to go with, just keep in mind that the more you tension the backstay, the tighter the headstay will be.
Also, the backstay pulls the mast back which induces aft rake to the mast. This will add weather helm where as forward rake will induce lee helm.
On some boats, the backstay is always being adjusted because this is a wonderful device to tune the whole rig while out sailing. The controls are in the cockpit where they are easy to manipulate. Don't be afraid to do a little trial and error testing to find out exactly how you like your backstay set.