I've noticed that when I'm hooking app-specific classes and try to store their instance variables, whenever I access them from somewhere else in my code it's as though the variable never got changed.
For example:

id instance = nil;  

%hook SomeDumpedYoutubeClass

- (void)regularlyCalledFunction
{
    %orig;
    instance = self;
}

%end

... somewhere else in a personal class or in another hooked one like Springboard's:

NSLog(@"instance: %@", instance);

Which gives the output: (null)
And of course if I try to call a method from SomeDumpedYoutubeClass with instance, it doesn't get called.
Yes, I do have the app in my plist.

Anyway to work around this, or am I doing it wrong?