You don't really improve your trot lengthenings by working directly on them, per se. You improve them by doing exercises which increase the engagement in your horse's hind legs - half halts, the almost walk exercise, etc. When you have more engagement in your horse's trot, try your lengthening again, and it is likely that it will be improved.
Always use both reins together to steer, especially when jumping. Using one rein only turns the horse's nose - but using both reins turn the horse at the shoulders, which means his body will more accurately follow your chosen line.
"Contact doesn't only refer to the hands, reins, and bit, but to the whole rider. A rider must give the horse contact through his entire seat. This means that his legs must lay gently against the horse's body, his seat must be balanced and supple,
Thinking about steering your horse from the middle of his body rather than his nose will help you to keep all of his body parts under control.
When schooling a green horse through any complex where the last element is a tricky jump (skinny, angle, corner, etc), it is smart, if possible, to school the last element first. This will set you up for success as you then jump through the whole complex, as your horse will