Read the Ingredients:
Reading the ingredients definitely helps. Identify these ingredients:
Sugar (AKA sucrose, glucose, dextrose, maltose, (high fructose) corn syrup, etc)
Mint flavor (shown as artificial and/or natural flavor)
NOTE-- Artificial flavor could be artificial sweeteners like splenda
Now, every good mint has either no or little sugar. If you do see sugar, good mints have it toward the end of the ingredients list. If it does have sugar, you won't get as much mint out of it. Make sure the artificial/natural flavors are towards the beginning, or else you won't get as much mint as you paid for.
Choosing the form (liquid and solid):
solid forms are dissolved by your own saliva. This can be bad if you get dry mouth easily. They are not as good, because there is no force to push them in to your mouth's skin, so usually the effects don't last long. That and they need something to hold them all together, so most companies use sugar.
Liquid forms are the best, they can get into the hard to reach areas, and can work if you have dry mouth. They can sink into the skin of your mouth, and don't need sugar. They are the best I've tried.
NOTE ABOUT 'LIQUID ICE'-- liquid ice is a gelatin outside with a vegetable oil and mint inside. These are small, but not so powerful, because of the vegetable oil contents. They are too small too, if you had two at once they work.
Price:
Of course, most good mints are 2 dollars per unit. I would get quality over quantity.
Size:
If you keep mints in your pocket, size can be an issue. Large tins like Altoids are bulky, and get in the way. Make sure you get something that will fit.
Flavor:
Flavor changes intensity. If you are looking for something mild try freshmint, or wintergreen. For more intense, try spearmint. For the ultimate, try peppermint, cool mint, or Cinnamon.
Recommendations:
Listerine pocket packs
Listerine pocket mist
ice breakers
ice breakers liquid ice (for people who don't want something so powerful)
any peeper spray on the market
So use these tips next time you shop for mints.


