Yes, your body does get used to them. At most, you might get 100cals a day burned via 'thermogenics' like Hydroxycut, or an ECA stack. in fact, if you decided to do anything, I'd go with a homemade ECA stack. Ephedra is not as dangerous as certain doctors would have you believe, especially if you don't take too much.
I would take them for no more than 8 weeks at a time with a month off between cycles. Which works well with a typical 2-3 month cutting cycle...if 3 months, don't start the fat burner til months 2-3 so you can use it to keep energy up and deal with those last few stubborn pounds.