Explore the citadel thoroughly after every story mission. Ambient dialogue results in missions that can lead to war assets.
Also, a quick tip that might help:
Any Citadel mission that has the prefix "Citadel:" will require that you get the item they need either from an N7 mission (scan everything you see), or from the Spectre Terminal.
Any Citadel Mission that has a prefix like "Ismar Frontier:" or any other star system means that you will get that item by scanning the star system as you're flying around.
Thanks for those tips!
The journal is pretty useless. lol.