Secret items
There are various weapons that you already have at your disposal, but there are also five secret weapons hidden throughout the game. The first of which is the throwing disc. You can find the throwing disc after completing the forest level--take the second route towards the industrial section and search the path to find it. The second weapon you can find is the hand cannon, which you will receive after completing the exterior hospital level. The third is the shuriken, which you can find on the third route towards Suburban after completing the cemetery level. Last is the spear, which can be found on the underground route at the beginning of the game, after completing the crash site.
Fountain of Health
This nifty little trick will make it so you never have to see the game over screen again. If you're in a battle and find yourself taking more damage than you feel comfortable with, simply bring up the map and choose the option to replenish your health. Your health bar will be instantly restored at the cost of your energy being completely drained for about ten seconds. Your energy is what feeds your special gadgets, camouflage and cannons, so as long as you can hold your own without those weapons, this trick will come in handy.
Fast game play
If you're trying to beat the game in record time, or just want to see how fast you can blast through the alien hoards, don't forget that the goal of the game is just to destroy all the aliens. If you find the human military units too much to handle, just sneak through or run past them. You do not have to annihilate these enemies to complete the game. If you're stuck on a batch of military soldiers that you don't want to waste time on, simply barrel through them, keep an eye on your health and use the fountain of health trick if they deal too much damage.