Tip 1: Be careful where you place your slowing cloud and perimeter destroyer. If they are too close to your planet they may become useless. Same goes for if you gamble on something turning up, say, in the corner of the screen and pre-emptively place a power-up there. If you’re wrong you’ll have to wait for the recharge.
Tip 2: Always fire off bullets or missiles (or both) into the distance, even if there’s nothing there. You can usually hit something or other just as it comes onto the screen and you have infinite ammo so you don't need to worry about recharge times, like with the special power-ups.
Tip 3: Upgrades come rarely enough for them to be considered worth taking any risk to secure them. Short of blowing up your own planet, you should always prioritise these. Unless they are a bonus points power-up, in which case: who cares?