I'm only a level 2 manager, but what I do is train the substitutes and reserves in order to boost the training bonus to 10 %. After that, I just play friendly games with them, usually 4-5 per day. If I do that regularly during the season, I basically gain enough rest packs to train a fast trainer from 99 % to 120 % for free. Well worth it besides adding some extra income and other packs.