Stephen King, among others, has said it, and I wholeheartedly agree: If you are going to be a good writer you have to read books, many books.

There are some writers who caution against reading the books of others while working on your own book, giving the reason that it might influence your own writing so that you begin to sound like the author you are reading, in other words, you may lose your unique voice. I will admit that this may possibly be a risk for a writer who is just starting out and who has not found their voice yet, but that risk can be greately minimized by not reading just before a writing session. And moreover, the risk is not so great that it is a good enough reason not to read, for how can one do something well unless one studies that particular field? Or to say it differently, how can you write well if you do not know about writing through having read what others have produced?
So my advice is to read a lot, and read widely. If you write poetry, read fiction, and non-fiction as well as the poetry of others, if you write fiction, read poetry as well, not just novels and short stories. Read within different genres too, for there is much to be learnt even from novels in genres far removed from your own.
You must find the time to read regularily. I will grant that this may not alway be easy. I know it from my own experience. I hope one day to do nothing but write, or do writing-related activities such as research, but until that day comes I have to balance a full-time day job with my writing, and time is a resource that is very scarce. And yet, I know must find that time, and I do find that time. Not just because I know that writers must read, but because I love reading and it is my favourite pastime. I would take it a step further and say that nobody who does not love to read can be a good writer, for you have to have that relationship with the written word that fires your own creativity, and that relationship is one of passion.
But lets say you do like reading, and you know you should read in addition to write, how can you find the time to do so, if you are not a full-time writer? Simply said, you have to make the time, you have to be selfish enough to not just claim for yourself the time you shut yourself inside your own little world and tap the keyboard, but also claim for yourself time to read, which usually means “alone time”, not alway popular with those around you. Moreover, you should learn to grab those snatches of time that are not filled with any particular activity, such as commuting or waiting in line. Over days and months this periods will add up and before you know it you may have finished several books in this way.
No, you say, I cannot do that, I drive to work. Well, you can still learn from the writing of others even if you cannot read, because you can still listen to books. I love the fact that you can download whole books on to your phone or ipod and listen to them whenever you want. You do not need to lug large books around (thanks to Kindle there is no need for that anyway) and you can listen to your books while commuting, going for a walk, jogging, working out, doing dishes, or even cleaning the house.