What works for Monica and myself is to keep all our receipts; once every month Monica sits down, adds up everything, compares it with my monthly salary, and figures out whether we’re in deficit or surplus, and by how much.
If you do that every month for a few months, you develop a sense of your spending patterns, and you are able to pinpoint, without much difficulty, the things that you might well do without, which take you over budget. In normal times you will want to put aside a little every month, to add to the reserve against a rainy day.