The pick should be held firmly enough so that you don't drop it. That's all. Any tighter and you'll choke the life out of it, and your strumming will sound "stiff" as well.

Try holding the pick between your thumb and finger, then try pulling it out with your other hand. What you want to see, and feel, is a little bit of play in the thumb. It should flex but not let go. That's perfect.

When you strum, do not push the pick through the strings. You should let your arm fall (pivoting from your elbow) so it falls straight down and quickly across the strings. What you have to do is stop the fall of the arm just as it crosses the 1st string. If you are trying to push the pick through the strings, you will have to hold the pick tighter.

Also when you pick if you can loosen your other fingers. To achieve this, place your pick on the 3rd string and set your other fingers (2,3,&pinky) on the face of the guitar. When you lift that away from the guitar you will have what I call the "perfect pick position". If your fingers are curled into your palm your hand will stiffen, causing your pick grip to tighten.

Then start with your pick just above the 6th string and let it drop across the strings. Your wrist should not move and your hand should not change position. Just let it fall, but be sure to stop it as soon as it's done strumming across all six strings.

Then when you strum up, you should not change the position of your wrist or the pick again. Just brush up. You will probably notice that you can't hit all the strings that way. You don't want to. The up stroke is usually an "off beat" and should be weaker than the down strum.

Is that enough of an exercise for you? This is the way I explain it all of my students, most of whom are very good strummers.
i dissagree with almost everything you said.

1) most of my strumming and picking is from the wirst, not the elbow. which most consider "better technique" than from the elbow. from the elbow to me is too clunky and doesnt work well for fast strumming.

2) many would call the "perfect picking position" you mentioned as achoring. however, there is nothing wrong with just touching the guitar. but suggesting to "lift away" may cause some people to anchor. also, there is nothing wrong with curling your fingers. some of the best players do it. some have there fingers flared out not touching or curled (eg. joe bonamassa). eric johnson curls his fingers, but he also uses them to mute the strings when going to the bottom strings. bottom line, there isnt really a "perfect picking position". its prefrence.

3)telling someone to not move the wrist imo, is wrong. again, i find it clunky and when you go faster it doesnt work too well. im not saying dont use the elbow, but it shouldnt only come from the elbow. also, not changing the pick angle seems odd too. changing the pick angle can change the tone and make certain thing easier to do depending on the situation. imo, its all about finding out what works for you, and finding out when to do it.
I would imagine that if you seriously analyzed the technique of a hundred experienced guitarists, you'd get a hundred different interpretations of "how" to use the pick.

Everyone has slightly different body mechanics, after all, and the style of music that's being played has a lot to do with things as well. Doc Watson refers to "picking from the elbow" when he's really playing fast, almost at the same speed as you'd do tremolo on a mandolin.
And most players do that with a rather rigid wrist.....

Robert Fripp, in his "Guitar Craft" school and ideas, had a great deal to say on the proper mechanics of picking, but I see that his web-page has largely folded. Maybe folks didn't agree?
I do agree with maintaining the ability to articulate the pick....To "choke up" on it when you want a little bit of fingertip string muting (Al DiMeola referred to his "Mutola" technique) or exposing more for a more ringing sound...
As any decent guitar teacher/guitarist will tell you, there is no point telling people how to hold a pick. As the two comments above have pointed out, everyone is different and so hold their picks in different ways, if they use a pick at all.