First of all, you need to get educated about the 4C's and how each quality and color directly impacts prices. Next, shop around. Visit your local stores and browse through the inventory of online vendors.
Do your own price checks by comparing diamonds with similar qualities (apples to apples).
One common mistake I see people make is when they start sizing up a 1 Carat G color VS2 Excellent cut against a 1 Carat G color VS1 diamond with Very Good cut and automatically assume one to be a better deal than the other. However, this isn’t a fair assessment since the 2 diamonds have different cut properties.
Also, when it comes to diamonds, you get what you pay for. Bargains never exist even if an advertisement screams 90% off. Fair deals do. Going online is the best way to get a bigger bang for buck and get better quality diamonds.