Decorations will increase the speed at which new customers appear at your door. Music will increase your experience points. Take note of the length of the records. The longer the song, the more bonus experience you can earn.
Faster sandwich seed growth
Seed growth is time dependent. Different seeds have different time requirements. You can instantly complete everything by advancing the time on your device by the amount needed to complete your harvest. Afterwards, set the time back to its correct value.
Growing
Water and fertilizer are important for the growth of your sandwiches. Maximize them in your pots to keep your sandwiches growing. Make sure this is done after you will be out of the game for some time. However, adding too much water will be bad for them. Remove pots by tapping the chair underneath them. You have to do this in order to replace them with new pots that are better than your current ones.
Selling
Take note of how each character reacts to sandwiches. Some will offer a higher amount of cash or decide to purchase a larger amount, however other characters will no longer buy anything. Get rid of low paying customers by selecting the "Get out" button when their offer is too low. Hide some seeds and water to avoid going out of business if you are busted and cannot bribe the police.
Upgrades
As you level, you will be allowed to purchase items from your clients, such as the bed, gun, etc., that will increase your crop yields. As you level, also purchase any new products that will give you an edge. For example, the pack of large water bottles is more efficient than the regular small bottle.
Experience...