Flushing with hot water is always a good practice prior to lubrication, to remove any salt, minerals and dirt that have accumulated in the cable housing. You'll likely notice a significant improvements just from that.

WD-40 displaces water well, but it's a really poor lubricant, so don't waste your time with it. Silicone will work, as will wax or Teflon-based cable lubes. Your local bike shop should have a good selection of suitable cable lubricants. It's easy to work lube in from one end or the other, just apply it, work the cable back and forth, then repeat as necessary.