I keep it very, very simple.
As long as my savings balance is increasing by $xxx per month, I am happy.
If it's more - great! If less - I look back through my online banking records and try and determine why, usually by identifying unusual or uncommon expenses of at least $50 (recent examples: plane tickets / symphony orchestra tickets / electric lawnmower / bike trailer).
I consider it a form of management by exception. =)
That said, I do have a mental estimate of what my bills, rent, groceries, eating out, transit, and miscellaneous spending will cost each month, so I do have reference values by category. However, I tend to find that some weeks my bills are greater, but I eat out one fewer time. Or I skimp on groceries because I bought too much last week, etc.
Whenever my income situation changes, or if I find myself consistently under-saving, I re-evaluate my income and expenses to determine what my $xxx monthly savings should be, and how much it is as a percentage of post-tax income. Presently it is hovering around 50%, which I am very happy with.
This may be an appropriate system for someone with relatively simple financial affairs, or who is more concerned with the outcome (achieving $xxx monthly savings) than with ruthlessly detailed optimisation, or someone who is easygoing (lazy *cough*). I am all three!