Coins
When you find an area on a wall that looks different, press up against it. It may open a hidden passage that hides an extra life or some money. Quickly collect the bouncing coins that drop from defeated enemies before they disappear, but make sure you do not get hit while trying to do so. Investigate chests and safes to get more money. However, some safes will attack you.
Store purchases
Purchase permanent life upgrades before anything else. They will always benefit you, unlike other temporary upgrades. Refill your hearts at a store instead of purchasing an extra life. Use the Heart Jar when facing a troublesome Boss. A small amount of energy will be restored if you die.
Various Tips and Tricks
In Kero Blaster you will receive weapons when defeating bosses.
You can upgrade your stats as well as the arsenal. It's better to focus on improving the frog's health first.
Permanent upgrades also take priority, since they will make your life way easier than a simple extra life.
Never forget to replenish your health before heading into a new stage.
Don't waste your time in grinding in easier levels, as the payouts won't be as nice as if you played later stages.
Secret passages that can be accessed by walking through specific walls. Make sure to always check everything out with care in each stage so that you won't be missing any treasure.
Weapons
Always make sure you are using the proper weapon for your current situation. New weapons are rewarded for defeating Bosses. Besides their range, specific weapons may be effected by the environment. For example, the pea shooter's range is very short when fired underwater, the flame gun is ineffective, the bubble gun's rounds float to the surface, but the wave weapon remains unaffected. Use the bubble gun to deal with traps and enemies that are positioned below you. Upgrade the pea shooter to the laser and use it to take out a Boss's armor.
Coins...