My shifters require a lot of force to shift. Specifically, it's the thumb levers. Shifting from the small to large chainrings in front, and from the small to large cogs in back takes a lot of effort and it's making my thumbs sore. I remember taking my bike to the LBS and they cleaned and lubed the shifting mechanisms but I don't know exactly how they did it. It didn't seem to take them very long, either.
Anyone have a link or something explaining the procedure? Thanks.

Older trigger shifters tend to gum up inside. Squeeze one of the thumb levers to create an opening and squirt some spray lubricant inside. Petroleum-based oil works best- WD-40 is mostly solvents, so it'll help for a bit but then go back to gumming up the works.

If the bike is more than 3 or 4 years old, consider replacing the cables and housing. They gum up, also, but it's pretty easy to replace them all. I did this recently, and the improvement in shifting was amazing.

Friction is your enemy here. There are three places friction will cause these problems: the shifters, cables/housing, and the derailers themselves. If the bike is older, dirty, or just not well cared for, take all the cables off. Clean them and replace as necessary.

Unless the derailers are really dirty or bent/broken, I don't think you'll have too many problems here. Just make sure they're clean. I also don't think it's too likely the shifters are the main issue. But I'd still clean and lube them up as suggested above. Most problems of excessive friction that I've seen are due to dirty or corroded cables.

Shooting WD40 into cables one at a time can tell you which part was causing the problem, as it will usually free everything up nicely. But most people don't recommend WD40 as a solution because it doesn't last long.

If not WD-40, what do you all recommend? Yes, I think this bike is due for some maintenance. She gets used in all weather. Right now she sports studded tires and it's snowing.

I have a few questions? what brand of bike is this and what lever components? is this a commuter, a weekend tear up the trails MTB or a bike that sees seldom use?

I do agree with above though that keeping your cables clean is very important.

for a lube/cleaner for the shifters I recomend Triflow or some other light oil based spray with teflon. WD-40 while good for getting things moving is not a good long term lube.

Anything petroleum-based, like CRC Ultra-Lite 3-36 Lubricant Aerosol Spray:

The bike is a Trek 820 mtb. It sees heavy use; singletrack, snow, mud, rain, etc. It's been under water - briefly - a couple of times. I was riding it when I hit a dog, flipped in the air, and broke my collarbone. It is my go-to bike. Almost everything except the frame has been replaced at least once.
Today, I lubed the cables and housings. The cable/hsg just up from the rear derailler was the worst one. The cable was dirty and it didn't slide well. I used Breakfree.
Rode 20 miles today on snow-covered hills through the local state park. Shifting was much improved. The day was beautiful. God is back in heaven and all is right with the world. Thanks everyone, for the help.

"I found God with a can of Breakfree." Boy, won't the guys at the office want to hear that one.

Yeah, sounded like a case of excessive cable friction.

Pretty much a given when pulling on the cable seems too hard, and a cert when the derailleur is slow to respond to it being released.

Tri-flo those bad boys! Squirt some all over the internals and let em sit overnight.. That should clear up any of the gunk thats keeping them from shifting smoothly.. Tried and true quick fix

If the cables are quite old and see a lot of use then I'd suggest new cable and housings. Over time the cable wears into the plastic liner in the housing and when that happens things get pretty sticky. On a heavy use bike it isn't uncommon to find that the cables are worn past their best performance within two or three years. Or sooner if used in muddy conditions.

get better components

jig-a-loo works well for my trigger shifters