I am always looking for ways to kill two birds with one stone. That's an idiom for doubling efficiency or cutting costs in half. Using the same batteries for my electric bike and power tools is an example. How would you like to do that every time you buy carbon offsets? When you buy offsets that go to wind or solar power, you're keeping CO2 out of the atmosphere by replacing fossil fuel, which is great. Everyone gets to choose, but like Jason has mentioned a few times, this approach not only helps reduce global warming, it also helps deal with its inevitable impacts at the same time. Me, I prefer the forest preservation and rehabilitation route, in part because it is more efficient, essentially killing four birds with one stone:

1. You are keeping carbon out of the atmosphere.
2. You are preserving a part of the planet's life-giving biosphere (and its biodiversity).
3. You are helping biodiversity to weather global warming by giving it a place to migrate to or from.
4. You are pumping money into developing nations that will use it to build their economies in ways other than cutting down their forests.

As for renewable energy, I think that's coming without me needing to invest further. By voting green, my tax dollars are already being invested in that, although not always wisely (mandatory use and subsidization of soy- and corn-based biofuels). I can also do things as an individual in that department, like riding an electric bike, recycling, driving a Prius, telecommuting, and living in a small, efficient urban home. By helping to save intact forest ecosystems (not trees per se), I have a well-rounded and diversified ecological portfolio. If someone wanted to pay me $10,000 per hectare to set my forest property aside in perpetuity as an old-growth forest conservation easement, I'd take it in a heartbeat, because, ah, a bird in the hand is worth two in the bush.