You should understand the dynamic of group-buying sites. Groupon.com, LivingSocial.com and others regularly post new deals with big discounts - usually 50% or better - for local businesses. The businesses get customers, and the middleman site gets a finder's fee. But the deal isn't a go unless enough people buy in. Also be aware you usually only have 24 hours to buy.
Check the reputation of the site you plan to use. There are more than 500 of these daily deal sites out there, and more are popping up all the time. Like any other online purchase, check into the reputation of the site you're giving your credit card number to. Check Better Business Bureau complaints and user reviews. Also check the reputation of the business you're buying a deal from. 70% off something like laser hair removal, a tooth cleaning or even a haircut isn't a "deal" if other customers say the business is bad news.
Comparing prices can save you more money. Just because a business is offering 50% off at a group-buying site doesn't mean that's the best deal out there. DealNews.com tracks local deals and compares the prices against other coupons and its previous offers. Also check the business's site for other specials and promotions.
Bring in friends. Most group-buying sites offer referral discounts for getting other people to buy that day's deal. If you like the offer, consider sharing it with friends. You could get a credit toward a future purchase, or even get that day's deal for free. Some sites like Tippr.com make the day's deal more valuable the more people buy it.
Check the secondary market. Prices for daily deals get even cheaper on sites like Lifesta.com and CoupRecoup.com, which let people buy and sell group-buying site deals. You may be getting a bigger discount, but check that the voucher is legit before buying.