to get them going - many things are mentioned in the indroductory thread... no?
lots of different ideas...

but to keep them going is tricky coz it's like drugs - u have to increase levels/stimulants every once in a while for the similar evect... if u keep doing it in the same way f.e rollacoaster first time is like oooooh-la-la...
but keep doing it for a while and not only that u 'll get used to it but even get bored of it...

in my opinion, it's best finding a variety of activities, using them, leaving them, coming back to them... following ur bliss...
and - the cleaner ur nervous system is - the stronger the response of ur body will be...

i got myself so cleaned up since i was pregnant and had baby that now, when i go out (1 or 2 times a months) im like a dog let from a leash - screaming of happines and excitement like im 16!!! and it keeps flowing for 2 more days, dont even feel the hangover (from 2-4 beers, maybe hehehehe)...

Hey Sean--if you're still looking for an answer... sounds like you guys are referring to tolerance... Moderating and depriving yourself of stimulus will "clean out your nervous system," so that you can be excited by subtler influences. Meditating, fasting, cleansing, sensory deprivation of whatever kind you can find, avoiding the usual stimulants--for as long as you can.