1. A speech synthesis system for synthesizing speech from text, the system comprising: a speech segment database configured to store a plurality of speech segments; means for determining a first speech segment sequence corresponding to an input text, by selecting speech segments from the speech segment database-according to a first cost calculated based at least in part on a statistical model of prosody variations; means for determining prosody modification values for the first speech segment sequence, after the first speech segment sequence is selected, by using a second cost calculated based at least in part on the statistical model of prosody variations, wherein the first cost is different from the second cost; and means for applying the determined prosody modification values to the first speech segment sequence to produce a second speech segment sequence whose prosodic characteristics are different from prosodic characteristics of the first speech segment sequence, wherein the second cost includes at least a prosody modification cost, the system further comprising means for increasing the prosody modification cost of continuous speech segments having a slope likelihood greater than a given value before determining the prosody modification values in response to detection of the continuous speech segments in the first speech segment sequence.