Summarize the following:
Place the bank behind you so no one can reach it. This will make them give you the money first. Every so often, instead of putting the money in the bank, slip it underneath you. Then, secretly take it out, and put it in your stash of money. Alternatively, you could take money from another Monopoly game and slowly add it to the pile throughout the game when no one is looking. However, be sure to mark these to distinguish it between the other notes. If you know that you will be playing Monopoly in a few hours, get to the game box, and slip out a bill from each denomination. Then, put the game back, and keep the money to use for the game. During the game, you can add the bills to your money pile, and no one will ever know. Make sure that you keep your money pile messy when you add to it, so that it is not obvious that you have an extra $500 bill. That is the bill that people may notice more easily. Any chance you have to steal a few bills from the bank, or from another player, take it. Act like you are busy on your phone, or distracted with something other than the game right after you do it to deter any suspicion. Instead of keeping your money organized, and neatly piled by denomination, keep it all together, and out of order. It is common for players to want to keep their money nice and orderly, but if you leave it messy, it will be easier to steal some money occasionally.

Summary:
Be the banker to stay close to the money. Steal money before the game begins. Take a few dollars when the other players leave the table. Keep your money pile messy.