When I didn't use the envelope system, all I did was a mental thing where I determine whether a purchase is "Necessary" or "Unnecessary". It worked but I realized that I underspent to the detriment of my lifestyle. Since then I switched to the envelope system and believe me it works. Not only does it compel you to save, it also compels you to spend on things so you won't be overly frugal.
I used a modified set of envelopes:
6. Transport (like when I need to use the bus or the train)
7. Credit Card Bill (I have stopped using my card and I'm just paying off existing debt)
8. Health Insurance
10. Petty Cash
Note that there are two additional envelopes, leisure and petty cash. I specifically placed a leisure envelope and put $30/week so I can compel myself to take a break and relax every weekend. There's nothing worse career-wise than getting burned out at work.
If I don't use up all of that $30 for that week, the balance goes to the petty cash envelope. I placed a petty cash envelope but it's downright IMPOSSIBLE to anticipate all of your expenses. There will be times when let's say you get a flat tire, or you have to go to the dentist, or it can be as simple as being suddenly invited to a wedding. Please note however that petty cash can only be used for necessities and not luxuries. For whatever luxuries you want, only the leisure envelope should be used which is transferred to the petty cash envelope at the end of each week.