I think its a good idea to train your weaknesses at the start of a session while your fresh.

Lets say you were working on a jab, straight right, left hook combo. Id work on that combo for three rounds using the system of shadowbox, heavy bag, shadowbox. People will probably say that dedicating three rounds to one combo is a little excessive but Ive found that it works well. Its important to balance it out but and make sure you get rounds in where your throwing your whole repertoire.