Buy powerups only when necessary as they cost money and make it that much harder to complete a level.
Buy powerup weapons:
Each better level of weapon will extract more gold upon each hit.
Be careful that you have enough money to spare to complete the next level before buying an upgrade. Each time you upgrade the King asks for a little less taxes the next few levels as he knows the upgrade will haul in more plus he runs the shop and makes money there too.
The King is adjustable:
The King is extremely greedy and his never-ending thrist for money.
He will watch your performance on each level and constantly bump up his expectation for the next level according to how much you scored on the previous level.
Use temporary upgrades:
Use the temporary upgrades such as Strength Tonic and Lucky Amulet wisely. On later levels these with upgraded weapons these items will allow you to haul in enormous amounts of taxes.