We've all been there. You sit down with a piece of paper, and write down rent, food, entertainment, etc. You struggle to figure out where to save $100 here, $25 there. Budgeting sucks, frankly, and what ends up happening is your savings/investments inevitably get squeezed.
Take savings/investing out of the equation. Figure out what you want to save (10%, 20%, whatever it is*) and budget from whatever is left over. It turns out if you 'pay yourself first' and live off the rest, saving is easy. If you do it the other way, it's almost impossible.
Budgeting in order to save/invest sucks. Take the money out of view, off the table.
* As an aside, the bigger this number, the faster you can retire. If you save the traditional 10% from a reasonable age (and invest appropriately) you'll retire with all your friends at 60ish years of age. If you save 20% from that same age, you'll be able to retire earlier - both because you're saving more and because you're used to living on less.