The horses barrel swings away from the hind leg that is going forward (when the left hind is going forward the barrel swings to the right) practice feeling that at the walk when you have it at the walk then practice at the trot. When the barrel is swinging towards the wall go up. As a bonus it teaches you feel which helps in timing for leg yields and more.
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A couple of people have touched on this, and I’ll reiterate: teach yourself to feel which leg is moving when. For now just focus on the front legs (though Milobloom’s advice is great!). You can do it at the walk and you can look down as much as you want. It helped my daughter, when she was younger, to walk over single poles on the ground because it was easy to see which foot was stepping over. And then try to keep track of which foot is stepping when as you continue walking. If you’re aware of what those feet are doing at the walk it’s relatively simple to pick up the correct diagonal as they start trotting since you’re just following a rhythm.
I’ve been letting some of my daughter’s friends ride my old mare lately, and it’s been interesting to watch the progress of the newbie riders. Some “get” the shoulder moving thing and some don’t. One can only watch for the front toe in that moment that it steps out in front of the shoulder, another can only watch the forward/back movement of the shoulder blade, and another absolutely cannot figure out how to “see” diagonals, but is okay with feeling/seeing the footfall at the walk and carrying that into the trot.
Best of luck in figuring out what works best for you!
Addendum…you will always pick up the correct lead if you post the first step of trot. You have to let the transition push you onto the diagonal. If you don’t post the first step, all bets are off. 
Yes! Learning to identify which leg is moving when will put you light years ahead in your riding. If you can identify when legs are coming off the ground at different gaits, not only will it make finding the correct diagonal simple, you will have so much more control of your horse.
The riders that look like they are in perfect harmony with their horses? They are giving aids when the horse can respond…because they know which leg is moving when. A horse can only change when a leg is unweighted. For instance, if you ask for a turn to the inside as the inside fore is leaving the ground, the majority of horses will start their turn naturally as they move away from the pressure of your outside leg. No reins required. That leg is unweighted and can can change from going on a straight track to swing to the inside to create the turn. Ask as that inside leg hits the ground? Your horse CANNOT respond, so you increase the aid until that foot comes off the ground again and they may or may not be ignoring you by that time…and you may be creating a dull horse. Same with the canter, you have to ask when the horse can actually respond correctly. Knowing where the horse’s body is helps you identify so much: if you are going to pick up the correct lead (heard someone call it ‘trotting on the correct lead’…just means the shape of the horse’s body means it will pick up the correct lead when asked), when the horse starts to spook so you can stop it by keeping the horse in the box between your legs, seat and hands, etc.
FWIW - I have always used peripheral vision to find my diagonals…I don’t move my head, or my eyes,…I just focus on the movement I can see in the bottom of my span of vision. I thought that was the only way I could get my diagonals…until I tested the theory. Turns out I don’t have to look at all because I’m aware of where the front legs are (fronts are easier than hinds). I don’t necessarily “feel” the diagonal, but I sense how the air is moving around the horse as it moves/how the space is being taken up by my horse at any given time…if that makes any sense. As the outside front comes forward, the air is moving more there and the space in that quandrant of our rectangle is full, while the inside leg has less air movement and the space in that quandrant is empty. 
Sounds totally cuckoo but it’s how I visualize it all in my head while I’m riding.
I’ve taught hundreds of riders (ages 7-60+) diagonals, and I never recommend trying to go by feel. Once they’ve been riding for a while and diagonals are second nature, then we get into that a bit, but when you’re first learning, just look!
Some people understand diagonals very quickly, some people take several lessons. For the ones who struggle more, I have a few suggestions:
(I’m assuming you already know to be “down” with the horses outside leg is back, and “up” when it’s forward, but struggle to correctly identify. I’m also assuming you know to sit 2 beats to fix it)
- Start at the walk, and say “up, down, up down” with the horses leg. Have an instructor of friend check you. You can even post at the walk to try this.
- Once you’re ready to try it at the trot, trot slower and really let yourself look at the shoulders going back and forth. Have your instructor or ground person tell you if you’re right or wrong, and then see what it looks like to be right and wrong.
- If you’re right, look at the outside leg to see what “right” looks like. Then look at the inside leg to see what “wrong” looks like.
- Go into 2 point for several steps, go back to posting, and check again. Have your ground person or instructor check you.
- If you’re REALLY struggling (99% of my students get it after several lessons with the above instructions), try going into two point and nodding your head with the outside leg. Nod down when it’s back, and as you’ve nodded a few steps then get your body to do the same. One of my adult students and I came up with this because she absolutely could not get her body to match what she was seeing as she was posting. But go into 2 point, and then her body was still and she could see it better.
Practice practice practice! Eventually you’ll get it! I don’t let my students move on to cantering until they’ve mastered diagonals so it’s good incentive. 
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Thank you to everyone who responded! After reading all of your comments I feel like I did much better in my lesson this week, I think my instructor only told me I was on the wrong diagonal twice. Usually its much more. Its hard to look down for me right now, as it does take a couple seconds for me to coordinate which leg to watch and which is going forward with me rising. Im sure this will get better over time.
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Figure the legs out at the walk before you ask for the trot. It is easier to do when the legs are moving more slowly, and they will just keep alternating even when you pick up a trot, so if you know where, eg., the left front is at the walk it is easy to continue to know where it is at the trot.
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Use your ears. Especially at the walk, a lot of times you can hear the hooves hitting instead of looking.
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If you are sitting (as opposed to two point) try thinking “down up” instead of “up down”. For whatever reason this made an enormous difference for me, I think because starting sitting it is easier for my brain to try and synchronize the down and let the up come naturally, rather than trying to synchronize up when I am starting from down.
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