Slow the game speed down...
I used to use an older computer and it went so slow I had time to get a good score. My all time high score is 1398. Part of it is luck because you won't always get the best Petpetpets appearing. However if you are at the last set of petpetpets or so wait and see if a higher point value one will show up. In addition if you time it right shooting the purple robotic petpetpet right you can wipe out 3 lines of petpetpets. Normally I just go for two but I have gotten 3 lines all at once. Those are some tips. Also your bonus score is 75 by default. For each petpetpet that you let slide past it subtracts 5 points. So obviously you want to limit that as much as possible and maximize your bonus score. Well I hope that helps some.