Those that have suggested a percentage have it right. It is a matter of changing your mindset. You must save and then live on the rest (assuming you have a surplus after required spending, which you say you do) rather than save whatever is left after you spend.
Calculate 15-20% of your after tax income and set up automatic transfers in that amount from checking to savings. That way you treat savings as a bill on the level with rent, utilities, insurance, etc. It does not compete with lunches, snacks, and misc.
Don't adjust your savings to your discretionary spending, adjust your discretionary spending to your savings.