Hi Roberto:
Perhaps you'd have to give more details about your program to know what's happening. I can tell you the way I do that. When key (up or down) is pressed the program stays in a loop till key is depressed. In this loop I set modify_add or modify_substract (depending up or down) and when I leave the loop I clear them. On the other hand timer0 is running. Timer0 interrupt adds or substracts if modify_* is enabled.
You can also measure the time during the key is pressed to change the add/substract speed. At begin you can add/substract each 50 interrupts, after each 20,...
I hope this can help you,
Regards
Alberto