A cheap and easy way to "overhaul" STI shifters is to shoot WD40 into the shifting mechanism. Sometimes this makes impressive improvements to shifting performance. Using the smaller shift lever releases the cable so that the derailleur spring can move the derailleur. Either the factory grease hardens and/or dirt gets in into the shifters and causes excess friction. The WD40 does a good job of loosening the grease and removing dirt. Replacing the shift cables and cable housing can also make significant improvements. Any excess friction along the path of the cables can impair shifting performance. When replacing the cables always replace the housings.

Al