You may not be able to. The ISP lines can't be loaded very much at all, again though this is irrelevant unless you can design your circuit so that whatever the reset line is driving can tolerate the 12 volts required to put it into programming mode, otherwise the chip will have to be removed from the system for programming.
Generally speaking if you're encountering these types of issues you need to move on to a larger chip with more I/O lines, if you still need small space then you need to learn how to work with surface mount components. There are MLF components with dozens of I/O lines that are smaller than 8pin DIPS. You need to make sure you're using the right device for the task, you'll spend more time effort and energy and possibly fail to adapt the device you're currently using to your needs.