There is a very easy way to guarantee you get a perfect score -- simply play on "PERFECT" difficulty. You automatically fail the level if you ever miss something that would contribute to a perfect score. Thus the levels become, simply
! = main level completed perfectly
!! = main level & retro warp level completed perfectly (So PERFECT score!!)
If you don't want to restart your save file and play through every level again, however, here's a general guide:
The trick to getting Perfect scores is simply a matter of getting all the gold, and performing all score increasing actions in a single run. In perhaps 90% of the cases, getting all of the gold will get you the perfect score, and there are only a few outliers where you have to make an effort beyond that of simply collecting the gold.
Here are the actions that score points:
- Kicking through a breakable object (Crystals, Stop signs, Boxes)
- Hitting a Springboard
- Bouncing off a Spring enemy.
- Reflecting a Bullet
- Picking up Gold
- Picking up a "+" object.
When given a diverging path with a choice (i.e., either hit a springboard or pick up a gold piece) always follow the gold and the "+" multiplyer, as these two objects are the biggest score boosts by an extremely wide margin.
When given a choice to either hit a points item or survive, chose the latter. Some points objects merely exist to present false choices - going after any item that will result in your immediate subsequent death is not required to score the perfect.