Well first it's about reacting, and knowledge more of their junglers rotation, if they invade and take your blue buff, you got and invade yours, without question, inform your team you are invcaded and counter invade, after which you go check your red, do your rotation, by the point your back near your blue you can go back into thier jung and counter golems or something. its all about knowing where they are.
what makes someone good at counter jungling is some form of movement boost, strong level 1, and and exicute. as nunu you can claim objectives with ease because he has all of them, starting q gives you a sustain execute making and invade easy.
what makes ammumu valnurable is that for him to jungle correctly he should start e, giving him very little that he could do to react to an ivade. when you amumu just assume they will be going for your blu, have the lane ward your blu,e and assume that you should steal theirs or go for your red. depending on what their jungler.
most melee champs, and champs that dont have mana start red, where ammumus and nunus start blue most the time.
i hope i could help