The main idea here is to give something up, something that will be missed, to serve as a sign of one's commitment to their ongoing spiritual growth. As a general rule, a one time act will have a relatively short-term effect. It's therefore preferable to give something up that will foster routine awareness of the sacrifice, or to sacrifice often.
The way I have personally practiced sacrifice for over a decade now is to give up items I would otherwise have regularly consumed for an entire year. I initiate my annual sacrifice on the Spring Equinox, but because of what I sacrifice, I tend to feel it every day. I have at different times sacrificed:
Coffee and tea
Snack foods, like chips, crackers, cookies, etc.
Potatoes, rice, and/or pasta
Other types of sacrifices depending on one's proclivities and lifestyle might include:
Television and/or radio
Gambling if it's a regular activity
Recreational drug use
Non-emergency driving (if reasonable alternatives like public transportation are available)
For the beginner, it's fair to start with something that's not too taxing, to see how it goes. If you find that you don't miss it much after a week or two, it probably wasn't much of a sacrifice, but there's no need to fret over that. Learn from it and make a more significant sacrifice next time.