Everything we are and everything we have, belongs to G-d. So what can we, mere mortals, give back to G-d?
When we say no to our evil inclination, we are giving G-d something back. As one X-addict wrote; "The only thing I feel I truly gave G-d in my whole life was giving up this addiction. For this alone, it was worth to come down to this world" .
If you think about it, there can be no greater sacrifice and gift to G-d than saying "no" to desires that take us away from him. For after all, even if someone gives a large donation to charity, or even if he brings a sacrifice upon the alter, he is still, in essence, giving back to G-d what really belongs to G-d anyway. However, the desires that we have, as human beings, do not belong to G-d. For G-d has no needs and desires, and he created our "desire to receive" out of nothingness, so that ultimately we will be able to receive from him all the good that he wants to give us. So by giving up our "desire to receive" to G-d, we are giving up a part of our own selves, and this is true sacrifice.