Food when I'm hungry, water when I'm thirsty, air... all the time.
Surprising as this might be to some, I say no to weed quite often. Either it is at an inconvenient time, or I'm already high and it's a wasted effort. I have no other vices.
By far the most difficult thing to resist is lending a helping hand. Particularly if I know I can help, then I see no reason and therefore won't say no. Plus, I like the feeling I get from helping. So that's probably my thing.