There are three things to consider when choosing a winter warm hands strategy:
1) Duration of the ride
2) Outside temperature
Obviously, the longer the ride and/or the colder the temperature, the more challenging the problem becomes.
When it comes to passive (non-heated) gear, two things are important:
1) Amount of insulation
2) Wind blocking
A huge amount of insulation will become bulky and make it hard to operate the controls. I've found that a moderate amount of insulation plus effective wind protection is a good approach.
If your existing gloves are already pretty warm but not warm enough, you might try the MotoCentric Overtrek
rain overmitts. I have these and it is amazing how much warmer your hands will be in the gloves you already have. Yes you look like you have lobster claws but hey, you'll be a toasty lobster!
They are just a thin windproof layer.
If your current gloves are not warm, I can recommend Icon Patrol
waterproof gloves. Because they are waterproof (actually, they aren't but most "waterproof" gear...isn't) they shed wind and they do have decent insulation.
If you want to ride for longer periods or colder temperatures, you really can't do without heated grips, heated gloves, or heated glove liners.
I use the Powerlet RapidFIRe
glove liners inside my Icon Patrol gloves. I bought the gloves one size larger to allow room for the liners. The kit is about USD $100. They heat the back of your hands and between the fingers.
Heated grips are great but to be honest heated glove liners are better. You have to squeeze the heated grips to get the heat to flow and they do nothing for your fingers when they are using the clutch and brake. If you can afford both heated grips and heated liners...that is the perfect situation: heat on palms and on backs of hands / between fingers. You can ride for hours...something else will get cold before your hands complain. Here is my thread on the Bikemaster
universal heated grip kit. Heated grips are great though for that shorter ride when your hands feel a chill and you're not wearing your warmer gloves or heated liners. You just click them on and get enough heat to finish the ride in relative comfort.