There is one tool that writers must employ to be successful. I am not going to waste any time embellishing this: the single most important tool for any writer is ORGANIZATION. Whether you're writing a book, novel, story, poem, article, sending agent queries or creating a blog post, there is nothing more important than getting organized. This doesn't just mean having a tidy desk; rather, it includes everything from timing, scheduling, planning, and outlining.
If you do an online search to find your favorite writer's process, you will most likely find that he or she sets a set amount of time daily, weekly, or monthly to writing. This means that you will have no disruptions during this time. For me, I lock myself into my office and my family knows not to disturb me during this time - this is work. I also leave my cellphone outside of my office so that I will not be disturbed. Trust me, this can be difficult at first, but it will pay off in the end.
But it is not enough to have time and space set aside; planning and outlining is also key. Without a plan, if you do stream of consciousness-type writing and plan to publish whatever comes out on that page, you run the risk of creating inconsistencies in your work. Of course, stream of consciousness writing has its place, but to me it is in the warm-up or practice stage of writing. It's like what doodling is to an artist. Sure, you may get some great ideas out of this, but they will be raw ideas that need refining. An outline will help your writing flow better, and it will help prevent holes in your story, which is incredibly important.
Keeping careful records of each character in each story (use character sheets - I have some available that you can use if you email me), each setting, location, and a graphic timeline of events and parallel events in your story. This is important even when writing nonfiction or memoir, because inconsistencies happen even in those mediums. And if you're into writing poetry and you haven't realized the importance of planning and outlining yet, then you need to get on it asap.
Additionally, it is very important to keep careful records when querying agents. Because each agent has his own taste in literature and each work is different, it is vital to keep a separate list of potential agents for each work. Also, you must keep track of which agents you've queried and when, and based on the information on their website, when would be a good time to follow up on your query. Keeping a database (I use an Excel spreadsheet that I can email to you if you contact me at MalaTobe@gmail.com) will prevent querying the same agent twice with the same work and from contacting a queried agent too soon.
Being organized will not only make you a better and more efficient writer, it will keep your writing tight. The important thing to remember here is that, as the old adage goes, "if you fail to plan, you plan to fail."