Step1
Get up with the times!
Over the years, various tools have come on the market to help with finding wood studs inside walls.
Early devices relied primarily on a magnet on a pivot (like a compass), which would be drawn by the metal nails used to attach wallboard to the studs; or, in the case of older, plastered walls, I guess the magnet would be attracted to the nails that held the laths to the studs.
These devices were inexpensive, but not very reliable, at least in my experience using them.
Step2
Understand the technology:
Modern, electronic stud-finders work on detecting density variations inside the wall.
When passed over a section of wall which contains a stud, most of these devices produce an audio and visual signal when the density changes, thus pinpointing the location of the stud.
I recently purchased one of these units, and I am quite pleased with its accuracy, so far.
However, these, too, can be deceiving, because they may also detect other objects behind the wallboard, such as nails, wiring, piping, and so on.
And, they may NOT detect metal studs with adequate accuracy.
Thus, some common sense needs to be practiced as well.
Step3
Helpful hints:
Here are a few common-sense tips that should help in many cases:
In older structures, studs were typically no more than 24 inches apart.
In newer structures they're typically no more than 16 inches apart.
Most, if not all, inside corners of walls begin with a full stud exposure behind the wallboard, in both directions.
Light switches and electrical outlets are usually (though not always) attached to the sides of studs.
Ceiling light electrical boxes are usually (though not always) attached to ceiling joists, which may (though not always) correspond to stud locations, at least on outside walls perpendicular to the joists.
(If you are relying on that last one, you really do need to buy an electronic stud finder!)
Step4
If all else fails:
Of course, if all else fails, there is always the "tapping method", where you merely place your ear against the wall, and gently tap-tap-tap your finger or a hammer across the wall to detect density variations in the sound of the tapping.
Where no stud exists will sound more hollow, and where one does exist will sound more muffled.
There...now wasn't that pretty easy?