Avoid the Overly Attentive Clerk
Can I be fat by myself for five seconds?! Thank you!
Avoid the Under Attentive Clerk
I refuse to track you down and ask for another size up. And then need a totally different pair. I can't do it. I won't!
To successfully endure trying on jeans in a private, timely matter: find the pair that interests you, grab twenty different sizes, and then find another pair and grab twenty different sizes of that.
Cry
Let it out. If you keep your anguish inside, you're going to have an aneurism and possibly freak out on someone undeserving.
Bring a Friend
Misery loves company. And if you're friend is there for moral support, you have someone to freak out at.
Put a Patch on it
So you found the perfect jeans. You're legs are lookin' tight and you're ass is lookin' wtf is written on my ass? Does that say "Naughty"? Don't freak out it's fine. There's a simple solution.
Learn how to sew

Put a patch on it.

Now you're the only one with this pair of jeans ;) How chic of you.
Learn How to Rock the "Douche" Look

Holy crap these jeans make you look dooooope! But they're $200 and they're like already ripped up...Take a breath, and just rock the pre-ripped jean douche bag look. Learn to love it. This is your life now.
And if you just can't do that, good thing you learned how to sew!

Beautiful. Done deal.
Know you're Measurements
You say you're a 29, but the clerk measures you at a 27. Trust him, he's the clerk and he just measured you. FALSE. DO NOT TRUST HIM. In fact, do not trust anyone. You have been doing this for years now. Take the 29 and run.
Settle

Never, ever, ever be afraid to settle. Don't you dare be afraid to settle! So what if the top button doesn't close. It's not like you wear crop tops everyday!
If it really bothers you just go on a diet, plan a work out regimen, and lose ten pounds. Easy enough.
They'll probably stretch out in the meantime. Or not whatever.
Maintain
If and when you find the golden pair that fits perfectly without adjustment, buy ten pairs and keep buying them for the rest of your life. If that company's stock goes down, buy it out. If you can't afford that, create a fundraiser, collect donations, save the company - and save yourself from jean shopping for the rest of your days.
It's easier this way. Trust me.