To improve all of these areas and improve topline work a lot on trot poles, stretching, icing you're horse long and low by pushing your horse back up and your horses head down and slightly in so he arches is neck into you-make sure he has a good forwards pace when doing to also allow him to use his hindend and back!! Also, work a lot on transitions-alot of downwards transitions, and also befor and after rides when your starting and ending trot around on a very loose rein and really push your horse and forwards ,asking him to relax,,:)
while doing flat work, work on making your horse round and getting your horse "on the bit". also massage the inside rein and keep steady contact in the outside rein to keep your horses head down; this will build muscle
There are a few things you can do to encourage muscle development around the neck, wither & shoulder areas.
1. Carrot stretches - getting your horse with a reward to stretch as far to both the left & right, as close to the flanks as you can get. Then underneath their belly without cheating (bowing) & as high as it can stretch it's neck to the stars.
2. Encourage long & low movement. You can do with by lunging with an aid such as the pessoa trainging system. It will encourage them to use their back ends & travel through their paces.
3. Lots of long rein riding, allowing your horse his head helps too!