I just finished dealing with the Cerberus and my assets are pretty low. I want to improve the odds, but when I got to the galaxy map, there was absolutely nowhere to go, no possible places to complete any quest. Does that mean that whatever I have when starting the Cerberus mission is all I'll have in the final?
According to this answer, there is no way to do any other missions after the attack on the Cerberus headquarters, effectively locking in your War Assets. You should have been prompted by General Hackett if you are sure you want to go through with the mission.
One thing that has usually been a staple with this series is that you will have your night with whoever you are romancing right before the Point of No Return to signify that you do not know if you are coming back.
The only way to get back before this point is to finish the game and you have the option of being returned to a point where it is before your assault on the Cerberus headquarters.
Play a number of rounds of multiplayer, get your effectiveness up to 100% or close to in order to max what you have.