Your main concern in the first two episodes will be the Energy problem: If
you buy the third weapon too early(like me), you will have Energy shortage
with your first and second ships.

A trick that helps to conserve energy: There is usually a Immortality bonus
in a level, when you are in Immortal mode, crash into your enemies, crashing
is by far the most powerful attack you can ever "perform", and it is both 
Energy Efficient and Cash Efficient since it cost nothing.

Anti-Laser driod should be your first priority, since there are little 
missile users in the 1st and 2nd episodes. But as you advanced to the 3rd
or 4th episodes, there will be more and more missile users, so you will 
want to have AT LEAST two Air-raid driods. 
Bear in mind that Air-raid driods will be destroyed easily if you don't
have Anti-Laser driods to defend them.
Another spoiler about the secret weapon in case you can't get it firing 
continuously: 
The secret weapon consumes 500 plasma(cash) each shot normally, and 100
plasma each shot when you get "Super Laser" bonus.

*Your first droid should be the Antilaser droid. Although it's tempting -
 don't buy the repair droid as the first droid - it requires the ship to 
 stand still to repair it. But: without defense droids you will have to 
 move the ship around a lot, leaving no time for the repair droid to do 
 it's job.

*Sell the missiles early in the game to get some extra cash. Especially
 the regular missiles are pretty useless.

*If those green swarm-pods annoy you: trigger them while they are still
 far away, then retreat the ship to the bottom of the screen. This makes
 it a lot easier to shoot the swarm ships, since they cannot surround you
 so fast.

*Turrets, while on the ground, cannot be hit by most weapons. They will 
 eventually raise, but you can also trigger them earlier by just moving 
 the ship close to them.

*Skip the second weapon and save the money for the third one. Probably it's
 one of the best in the game (especially, since it can attack turrets on 
 the ground). Unfortunately it has a high energy consumption, so you will
 need to upgrade the ship, too.