I can't speak to "best," but I suggest reading Style: Lessons in Clarity and Grace by Joseph M. Williams, which crystallizes lots of non-trivial components of "good writing." (The link is to an older, less expensive edition which I used.)
I'll also second "write a lot" and "read a lot." Reading closely and with purpose in mind will speed up the latter (as opposed to the default of throwing books at your brain and hoping to pick up good writing by osmosis). Also, read good writers.
Granted, writing is not very effective. But some of us just love writing...
Earning to Give Writing: Which are the places that pay 1USD or more dollars per word?
Mind Changing Writing: What books need being written that can actually help people effectively change the world?
Clarification Writing: What needs being written because it is only through writing that these ideas will emerge in the first place?
Writing About Efficacy: Maybe nothing else needs to be written on this.
What should we be writing about if we have already been, for very long, training the craft? What has not yet been written, what is the new thing?
The world surely won't save itself through writing, but it surely won't write itself either.