I've always sucked at guessing distances, especially anything over 50 feet,  and I want to start practicing long range shooting with iron sights.  How can I improve my range-Fu? I know you can get spotting scopes/rangefinders that spell it out for you, but I want to improve my ability to estimate distances without external equipment. Any suggestions or tips?

Go to the local high school football field.  It is easiest to get used to the 50 & 100 yd increments.  See what different sized objects look like from different ranges(ie kids, dog, etc.)

Go to a KD range.  

Practice.

Go get a laser rangefinder. Guess the range and see how close you were.

Do it a lot and you'll get better.

Personally, growing up a farm kid, have always been pretty good at guessing distances and times.

WIZZO

I like the idea of the local high school field...

When I used to shoot bows I would take and place a target and then walk away taking choppy, long, and different steps drop an arrow, then do the same to get to another spot...then guestimate the range and shoot it.  Just what people wanted to see, a drunk on the local high school field shooting arrows...

I really recommend you don't do this with an AR...

Point being hit the local field take a few baseballs/footballs, toss em out  at various distanced and estimate the range, then verify with the yard markers.

Used to do it just walking around too.  For example, walking around the mall, pick a store front, kiosk, group of people, guess the range, then pace off one yard per stride.  Should get you within a few yards of the true distance...

I practice that at the range.  When people go down to check/change targets I make mental note of there height, the size of their vehicle, etc.  at different distances.  Most people drive down to the targets on the rifle range  I go to because it's 500 meters  so you'd have to walk a klick before the range could go hot again.   Naked eye of course.  It would be pretty rude to scope them out.

For iron sights you can set targets of known widths at 100yds and determine the MOA size of your front post. Knowing that, you can use it for rough range estimation on distant targets, as long as you also know the size of the target. Same can be done for the rear peeps but they have a very wide field of view, are only usable on big targets, and subject to variations due to eye distance from the peep.

You can also learn with practice. Buy a laser rangefinder and use it to verify your range guesses to various objects.

determine the MOA size of your front post

could you elaborate this a little for me?  I have an A3 carry handle, so I know that the rear sight is incremented in 1/2 MOA, but I'm fuzzy on how to use MOA.... (i know thats supposed to be basic info, but i forget...alot.)

Hold your thumb out in front of you. At ten feet it may cover something only a few inches wide. At one mile it will cover a much larger object. If your front post is 6 MOA wide it will cover just over 6 inches at 100yds. At 1000yds it covers 60 inches.

Looking at an object 18" wide at an unknown distance you see that your front post just covers all of the target. Guess how far away it is.

ah.... that makes sense. thanks.