To a certain extent, you need to remember where your important assets are. Beyond that, you can always use the empire tree (on the left-hand side of the screen) to browse your assets in each gravity well, or you can just zoom out and highlight the different planets to see what's there.
As far as military groups go, I just use control groups. I will select my primary task force and click control-1; now every time I click the "1" key, these units will be selected (double-clicking "1" will immediately center my screen on them). Using a different number (0-9 all work) for different groups, I can then cycle between each of my military task forces with great speed. This is pretty much the only hotkey you need in Sins, you can get away without using any of the others. I personally don't use fleets, I just use these hotkeys and manage each of my forces manually to maximize performance.
Once your comfortable with multitasking and managing your units, you should try turning off autocast. You can get much better performance by manually controlling your abilities, and not only that but then you can start budgeting antimatter on your capital ships, which makes sure you have enough for when it counts.