I've given this a lot of thought as I've moved towards a lower-impact lifestyle. The problem is that much depends on where you are and how you live in the first place.
The first thing to consider is long-range impacts. The single biggest decision any of us can make in terms of reducing long-term carbon (and indeed other) impacts on the planet is to NOT HAVE CHILDREN. http://www.theguardian.com/environment/2011/sep/27/not-have-children-environmental-reasons This is a decision I've made for myself, but I'm not sure how much difference it's made. Three different women in my life have moved on in order to seek a suitable sperm donor.
" in the United States, the carbon legacy and greenhouse gas impact of an extra child is almost 20 times more important than some of the other environmentally sensitive practices people might employ their entire lives – things like driving a high mileage car, recycling, or using energy-efficient appliances and light bulbs."
After that, much depends on where you live but, for most people in affluent countries reading this the main ones are, in order:
Transport: give up the car and quit (if you ever started) flying. The single biggest thing (after deciding not to procreate) is give up the car.
Housing: these tend to be individual things that add up, but it's worth seeing what you can do without or finding less polluting alternatives. Consider also embodied energy. My electricity use, in spite of the fact that I buy from a supplier who produces all their own electricity from renewables, is as low as possible, because everything I don't use is also a lower strain on non-renewables.
Diet: this is quite messy but, as a general rule of thumb, the higher you go up the food chain, the greater the impact. Red meat is worse than white (beef being worst of the lot, even if it is grazed (stall-fed animals move around less so, however appallingly inefficient it is at producing protein (not to mention cruel), grazed animals are not a lot better in terms of energy conversion: http://www.monbiot.com/2015/12/22/sacrifice/)). A vegan diet is better than a vegetarian one, a vegetarian one is better than one based on, say, chicken and pork, and all these are better than eating beef. Note that cheese is surprisingly high in terms of carbon:calorie ratio. If everyone went vegan we could cut agricultural emissions by two thirds and save the lives of 8 million humans a year by 2050: https://theconversation.com/going-veggie-would-cut-global-food-emissions-by-two-thirds-and-save-millions-of-lives-new-study-56655 Yes, meat really is murder.
Probably the biggest thing we can do is get involved in the movement to keep fossil fuels in the ground. In terms of our personal lifestyle, my afraid I'm not the right person to ask. And most of the sites and resources that help you analyze your carbon footprint look at your admissions but not at your sequestration. One of the big things we can do in the wealthier countries is greatly reduce our levels of consumption, which I suspect most members of this site are already well on their way towards. Likewise reducing energy use, increasing energy efficiency, and so on. Some afraid I don't have a great answer for that but it is a question I get a lot and I' ll need to get a better response:)
To add some numbers to this question, current estimates of the total carbon locked up in land vegetation is about 600 gigatons, maybe double that in the soils[source] with roughly 40-50% of that vegetation carbon being stored in rain forests[source]. At present some 11 percent (1.5 billion ha) of the globe's land surface (13.4 billion ha) is used in crop production[source] Increasing the world's non-rain forest land biomass 20% by planting trees would be really impressive and would provide a benefit of locking up an additional 60 gigatons of carbon.
While 60 gigatons of carbon is a massive quantity requiring a world-wide effort, that effort would only offset 5 to 6 years of carbon emissions from fossil fuels (~10 gigatons/year, and rising).