You can kill a baddie with your ball or by log-rolling him. Save the ball for emergencies and use the logs as much as possible.
It takes 16 dots to regain your ball after you throw it.
Each bounce of your ball against a wall increases the points you can get if it hits a baddie, up to 3000 pts max. Somewhere on every board, is hidden a 'letter', that when surrounded by your path, will bring out the 'E-X-T-R-A' guy if you step on it. It changes as time passes so use this to get the letters you need to come out so you can spell extra as fast as possible.
The extra guy will also come out if you kill or log-roll enough baddies. If you use the hidden letter and a log, you should be able to get 2 EXTRA letters per level. This will earn you more free Mr. Do!s as fast as possible because the higher levels will get tough.
Stepping on an EXTRA letter will not only bring out the EXTRA guy and his entourage, but the baddies on the level will slow down to half speed. This makes it easy to avoid them while you lure the EXTRA and his ghosts under a log.
Use the logs as much as possible. They kill the most guys with a single log to get the most points. If you can lure 3 or 4 baddies into the path of a log, they will not move out of the way once the log is pushed and you kill them easily. Walking under a log knocks out the post holding them up so get out of the way if you do that. You do not have to push the log from above to get it moving.
A log will also roll if it is hit by a flaming/flying snake/dragon. Be aware of this so you don't get crushed!
A level ends once you get all the baddies (or get all the dots). Use this to your advantage. If a log is going to run over the last baddie, let it run into you too. If all the baddies are in line with a log, you can start it rolling, let it roll you and then the baddies will freeze in place for the log to roll them too. Your life will be restored at the end of the level if you get all the baddies!
Mr. Do! slows down going up a step and speeds up when going down them. Use this to your advantage if you are being chased. The baddies don't speed up going downhill so you have the advantage.
Don't waste your time gathering dots or surrounding them to get pineapples. There are more points in log-rolling and multi-bounce hits of your ball. Collect dots only to restore your ball or expose the EXTRA letter. You should be able to get about 20000 points per level using the above techniques.
Try to make concentric circle patterns from the outermost edge in, 'eating' the outermost cherries, dots or whatever each time. This will allow you to get a ton of pineapples, which score the most, and will get you a good score in early stages before it gets difficult.