As with all tower defence games, carefully consider where you place your towers. Find spots where your defences can attack aliens more than once such as switchbacks or corners.
Sentinel orbital weapons like the homing missiles and ion bomb are extremely powerful and great for saving you in a pinch; however, you can only use them by generating enough energy, which accumulates slowly during each wave.
Upgrades are vital to success. Balance the purchase of new towers with credits spent on upgrading your existing defences. Often upgrades more than double a unit's attack power.
Don't spend credits purchasing a slot until you have a tower or ability to fill it.
A fast-forward button lets you speed up the time between waves since you're unable to control the start of each wave.