Men: What is a succinct, clear, Halachically complete line to tell a barber what not to do when cutting your hair?
Find a friend and ask: וואו איז א גוטער שערער
When you arrive ask: איך וויל זיך אפשערן
If you're still in doubt mention: שערט ניט פון די פאות
(See "Say it in Yiddish" by Uriel Weinreich and Beatrice Weinreich page 132)
If this doesn't work then its a little harder to give a one sentence explanation but I think it can be done.
I would suggest saying something like this:
"Please do not shave/remove/cut too short/touch the hair from behind/in front of my ear to the temple/back of my hair line and below." While doing so touch the various points to help illustrate your intent. Then watch like a hawk.
The Shulchan Aruch (Y.D. 181:9) gives the dimensions of the peyos. As I understand it it says that the area extends from the hair opposite the forehead down past the ear to the joint where the lower jaw begins. The entire width is included in the peyos which I presume to mean until [at least] in front of the ear.
Nitei Gavriel - Chinuch Yeladim, Halachot & Minhagim has a section, I believe chapter 6, which discusses the minhagim (especially Chassidish ones) associated with peyos. In the back (here and here) it has some nice diagrams illustrating the different areas (which I need to study further). As I recall there are two areas really in in question.
One is how far up on the forehead: One appears to be the furthest point forward of the hair line, the other is the point where the natural hair line dips furthest back (which is higher on the head).
The second is the back edge of the peyos: one of which was placed in front of the ear the other half way past the ear (or more likely behind where the ear attaches at the head).
Additionally some arch the line between these two points up.
(It really helps to look at the pictures)
I think this is a sufficient explanation to keep one out of too much trouble but one needs to speak to a Rav who is familiar with the basic halachah and your own customs.
I've found the best policy asking to "leave the sideburns intact" and then taking care of the minor job myself at home. I realize this is a non-answer, but it has proven more successful at achieving the desired result of not appearing to have cut my hair in a forbidden way than other forms of instruction.
It seems no one has discussed how long the hair actually needs to be.
The Mishna in Niddah 6:12 states
שתי שערות האמורות בפרה ובנגעים והאמורות בכל מקום כדי לכוף ראשן לעיקרן דברי רבי ישמעאל ר"א אומר כדי לקרוץ בציפורן ר"ע אומר כדי שיהו ניטלות בזוג
The [length of the] two hairs regarding the Red Heifer, the Tzara'at affliction, and in all places is enough to bend their top to their root, according to R Yishmael. R Eliezer says they must be long enough to catch with one's fingernail. R Akiva says they must be long enough to cut with a scissors.
The gemara there comments (Niddah 52b)
אמר רב חסדא אמר מר עוקבא: הלכה כדברי כולן להחמיר.
Rav Chisda said in the name of Mar Ukeva: The Halacha is like all of them in the most stringent way [i.e for every instance pick the opinion that is most stringent].
So, we have to decide which length is longest and how long it is. According to here the longest length is the first opinion (bending head to root). I have never done any personal research, but according to this site and confirmed by a number of people I have spoken to about this, the minimum length necasary to satisfy all opinions is a #2 clipper guard. So as long as you are getting your hair cut longer than that, you needn't worry about what area of the head they are trimming.
(All translations are mine)