Sign contracts as late as possible. When they sign a contract their wage increases. If you still want to sell the players later on in the season their wage is low, so it easier to spot a good trainer based on the wage. In that case you haven't lost the signing bonus and you have better selling material. In my league (lvl 16) a wage can go from 2 mln to 6 mln, which is a lot of money if you multiply it by 18-22.