New or Used
Entire sets are worth more than individual pieces. You should also consider whether the flatware is new or used. Naturally, silverware that is still in the package will almost always be worth more than a set that has been used.
Monograms and Patterns
Monogrammed silverware is worth significantly less than a set that has not been monogrammed. Patterns also affect the value, depending on their popularity.
Size
Silverware comes in a variety of sizes, including luncheon, dinner, and continental (listed in order of increasing size). Larger sizes require more silver, which increases their value.
Hollow vs. Solid
Similarly, flatware sets that have hollow handles will be worth less than those that are made from solid silver. You should also examine the knife blades, as they are sometimes made of stainless steel and can reduce the value.
Availability
Finally, do some research to find out whether or not your silverware is still being manufactured. If it has been discontinued, it is most likely worth more.