i have the Shimano STI brake / shifters and they gum up and don't shift anymore. i took it to my LBS for other work too and it was fine for a couple months. Gummed up again and i took it back and they fixed again and told me the fix is WD40.

So they're gummed up again and i want to do it myself. I sprayed all over, made a mess and only freed up one shifter -sort of.
Where do you spray? do you squeeze the brake and spray from the top? or through the hole on the side? Do you drain the can or just a few blasts?

How long should this fix last? are they shot? they get very little use - 20 miles a week through the warm months

Peel the rubber back a little and use the straw to blast WD 40 in any opening you can.
Better still, turn the bike upside down and use the straw to shoot it into the works.
Use lots, clean the levers to remove the excess WD40 from the outside.

Shifting problems caused by worn or dirty shift cables and cable housings can act similar to gummed up shifters.
The cables and housings should be replaced occasionally. The short housing at the rear derailleur is most critical.

Recently I had shifting problems in the front due to dirt and grime in and around the pivots. This could also be the cause of sluggish shifting at the rear derailleur. I soaked the FD in mineral spirits and sprayed WD40 on the pivots. Problem solved.

This would get my vote as most likely cause.