Being spontaneous is not always a bad thing, spontaneous plans are oftentimes the most rewarding. However, it can also lead you to spend a little more than you should. A good rule of thumb is to make a list of needs and wants.
This can function either via a physical or digital list. Write down your grocery or shopping list prior to your visit to the mall or store to better prepare yourself amidst the temptation of buying more than you should.
However, it is also important to understand that severely limiting yourself can also act as a trigger for splurging, creating pent-up desires to indulge that can potentially break loose which will eventually manifest itself as overspending.
Keep that in mind, and reward yourself from time to time, your saving experience should not weigh you down, instead, it should be a sustainable and ongoing process.