I stand up when the outside leg comes forward, at least that is how I was taught. I dont always pick it up right away, but look down pretty much immediately to ensure I dont put the horse off balance. After a year of riding lessons, you'd think I wouldnt have to look at the outside leg anymore!
Have someone tell you when you're on the correct diagonal, then look down to watch the outside shoulder - stand up when it goes forward, and learn what that looks like. Once you can identify it by looking, check and make sure it's correct, then spend time practicing - learn what it feels like when you're on the correct diagonal and when you're on the wrong one. The best way to tell is on a circle rather than a straight line, because it is harder for the horse to balance and move properly if you are posting with the wrong shoulder - it should feel rougher/bouncier and as if you are being pushed more towards the outside of the circle, if you're on the wrong diagonal. If you're on the correct diagonal, it will feel as if you are moving nicely and smoothly with the horse in the direction of travel.
Let your horse stretch and walk forward. While doing so, relax your body (especially your hips) and see if you can feel the motion. Imagine what the horse looks like when walking and trotting. There is some side to side motion as well as the forward and back motion. At the posting trot, you want to "rise and fall with the shoulder on the wall" but the important part is to sit when the inside hind leg is under the horse (which is also when the outside fore is back and under the horse). We look at the front shoulder since we can't look at the hind end easily but the hind end is the important part.
When you think you have it, move up to the trot but sit the trot and feel it the same way as at the walk. Next, you can try to post but do so in either a circle or in the corner of the ring since it is easier to tell right from left when the horse is turning (because the outside legs have to cover more ground than the inside legs on a turn).
Your goal shouldn't simply be to be able to pick up the correct diagonal. It should be to know what is going on underneath you at all times. When I leg a horse into a trot, I can immediately pick up the correct diagonal in the first step but it's because I already know where his legs are when I start.
Practice. Then practice some more. Really try to visualize the horse and what he is doing while you feel what's going on underneath you. You'll get it but it takes time and practice!
When I leg a horse into a trot, I can immediately pick up the correct diagonal in the first step but it's because I already know where his legs are when I start.
This is how it is for me too, except that I wouldn't say that I really even "think" about where my horse's legs are. I just pick up the trot and am immediately on the correct diagonal every time. If for some reason I WANT to pick up the wrong diagonal, I have to really think hard about it (and it feels extremely wrong to post on the wrong diagonal if I am doing so to try to figure out an unsoundness or something).
Anyway, the OP really got me thinking! I honestly can't remember learning my diagonals (I guess I would have been about five years old at the time). So, now I'm sitting here trying to think about what I feel when I pick up the trot that allows me to start posting on the correct diagonal immediately. I guess what it comes down to is that I *do* know when the outside front leg is about to come off the ground into the trot stride, and that's when I start the "up" part of my post.
So maybe try doing a lot of walk to trot transitions and really think about where the horse's legs are in the moment where the horse goes from walk to trot. I wonder if watching horses pick up a trot might also help.
In the end, it really is a "feel" thing...hard to explain, but once you've got it, I think you've got it forever.
Coming down to the trot from a canter, I always pick up the correct diagonal; noticed in my lesson yesterday when my horse landed on the wrong lead that I picked up the wrong diagonal automatically as he came down to a trot. So for me, it seems to be easy to feel the movement and find the correct diagonal naturally if I'm feeling the canter. That then translates to sitting trot; I can now feel the diagonal much more easily after sitting for a few beats.