To eat an item, you must get it touch with its center, not only a few pixels. So be sure you have really eaten what you wanted, before turning back.
At first levels, the bonus does not make a lot of points and ghosts are not very fast, so you should concentrate on eating the most ghosts that you can.
After about ten levels, the ghosts are faster and the bonus makes more points, so you should try to eat the bonus when possible. Do not hesitate to eat an energizer when you can see the bonus, to not be disturbed by the ghosts.
In the same way, you should try to eat an energizer before going to the dangerous places (like near the ghost home).
When the ghosts are too fast, you should avoid to eat them if you are near their home. They could go back faster than you think.
Do not hesitate to stay in the same place for a few seconds, waiting for the ghosts to go far away from the place you need to go.
You can find cheatcodes from the source code. But be careful: if you make a new high score but have cheated, your high score will not be registered.
Last but not least: Have fun !