Where I started with my horse was his jaw. Is your horse soft in his mouth or does he have a tendency to clench or grip or "lock" ?
My horse had all of the above, so a few times I'd encourage him to release his jaw in the same manner I would putting the bit in. Finger in the corner of the mouth, wiggle until they open their mouth, lick and chew. I made sure that he was comfortable doing that. Then I did it with a bit inside. I'd softly "play" with one rein as I would my finger, until he released, licked and chewed. Over time, I upped the anty and encouraged him to keep "soft" by flexing him at his poll when he was standing still. I'd start with his stiffer side first, and once he got that down, I'd go to his easier side.
I did carrot exercises, slowly building him up to where he flexes all the way behind him and can touch his own barrel with his nose. Down between his legs (remember this is over time, and done very very very slowly) and stretch out by holding the carrot piece out far in front but asking him to not move his feet.
Back to under saddle, I'd warm him up on a nice loose rein. First we'd walk around on the rail without any kind of contact or bit interference. Then we'd go into nice loose serpentines, circles, figure eights, and whatnot. I'd keep encouraging him to stretch down and soften that bit in his mouth. Back then I would comb the reins because that's the only way he'd relax. Now I just "sponge" the reins and ask him to move forward and down.
Hope that helps.
"Strength is the ability to use a muscle without tension"