What is your fittening programme at the moment? It all depends where you're at and what your aims are, and it's really involved. I think the only "tips" that would apply to everyone would be take it slowly, but also I'd say that musculature IS posture, lack of topline is really common, we just don't recognise it well. Equitopiacenter.com has two brilliant webinars on topline syndrome which are good places to start to understand the topic.
I'm not sure what you're currently doing but I'd say it is really worth investing in either in-hand or ridden (or both) biomechanical sessions to help ensure you are strengthening the correct muscles and they are using them correctly. There's no point making them strong in the wrong places as ultimately they'll just become compromised and weaker overall.
Also, most of my fitness work takes place out of the arena as I think it helps hugely - especially if you have hills and different surfaces
Horses Inside Out by Gillin Higgins are excellent books for learning about anatomy and function. There is lots of info on thr website and fb page too. You can register for free webinars. Lots of info on how to get fit, condition the body and be hands on.
Getting Horses Fit by Carolyn Henderson is a good book too along with From Warming Up to Cooling Down by Susan McBane are good additions to your library too.
Remember as your horse gets fit it will change shape so ensure that the saddle is checked regularly.