I volunteer at a local bike Co-op as a mechanic. I see sticky STI shifters regularly, usually 10+ year-old models.
First do the WD40 flush with the shift cables disconnected from the derailleurs. Work the shifters through their entire range. Then follow with lithium grease lube. This is a spray-can lubricant in which lightweight lithium grease is mixed with a volatile petroleum-based liquid. The liquid vaporises, leaving the lithium grease behind - in all of the right places.
Your shifter cables and housings should be completely replaced. This should actually be done yearly as a matter of course...
Finally, check the action of your derailleurs; they may be sticky due to corrosion or possibly they are bent due to an impact.