I think that it depends WHERE you are as a writer...I know for me getting started with creative writing as something completely new, signing up for a few creative writing workshops initially was helpful. A bit like doing parenting skills classes. Although I'd written professionally for many years in my vocational field (journal articles,masters disseration, conference papers) Id never atempted anything like writing fiction. I started this originally out of a sort of deep urge to express my self creatively - after comming out of a very challenging period in my life - the feeling was almost primal; quite "from my guts" infact. This was the absolute beginning for me and that was done alone and had nothing to do with being taught on a workshop or any course. But then I felt I did need something within which to frame what I was doing. Not having any kind of literary background(my father was an electrician not a famous novelist; my mother a cleaner - not some Pulitzer winning journalist) - unless you count a grade B in O level English in 1975! completing a few basic "creative writing workshops" was both inspiring and informative I found. But also then I realized what real writing is about - hard, soul searching slog working alone...though I'll come back to this later because its not just this, its far more than this infact- and- met some of these writing course junkies who were always seeming to be dreaming about the next course rather than actually about geting down to compleing a project of their own. I think therefore that workshops can be useful for learning the basics through writing excersizes - often these might generate ideas for scenes/charaters/a story. Endless worksops and short courses are not helpful in my view.
What I found helpful is being selective and choosing when you need to do these and what outcome you are expecting. The first course I did was six weeks long, two hours every Thursday night. It involved writing and then performing a 4 minute monologue. The next course I took about 6 months later was the excellent London CityUni 12 week evenning course in Short Story writing with Katy Darby. In this I was expected to complete one short story by the end of the course - which I did and it ended getting published in Decongested Tales and being read at their Foyles monthly short fiction event. A few months later I took another 12 weeker at CityUni - Intermeadite fiction writing - which was really more like workshopping with a small group. This gave me an idea about how to go about writing a longer piece and the level of skill required. Then I enrolled in the year long Graduate Certificate in Novel writing - a mixture of taught and experiential modules looking at various genres and reading alot( for me especially I developed at art of readng with a writers eye )and also encouraging a writing habit - a regular routine. The course also has links with the publishing industry who look at graduates for new talent.
I 've done also a couple of led writing residential retreats and would say these are useful for different people in different ways. Some use thm to overcome block or for inspiration or to see how the "masters" do it. But I found networking with other writers(as others have pointed out, writing is mostly something done alone)and having an environment supportive to write..whether its to produce new ideas or begin a new piece or move something started forward. Out of the two I did I produced two short stories again which were published(eventiually).
Joinng a good writing group I think can be extremely important - nothing beats airing your work and guaging its impact , although its important that you pick a group that is supportive and right for you. Now I've embarked on my first novel I am finding haing a mentor is really helpful to me. A montly check in, written and verbal feedback on new material Ive produced and small excersizes to stimulate new ideas for scenes, characterisation etc are all helpful.
But as Emma says - it really does always come back to you, producing new material and for this you need discipline and a regular habit. I personally cant write non stop all day for days on end. I just find it counter productive , I get bored and dont create .For me its 2 1/2 hours from 11am then STOP and pick it up again in the evenning for no more than 2 hours. That way I may produce 1000 1200 (sometime half that!) words but good words and am satisfied not by the word count perhaps but by the material.
The word count goes up over weeks and months.
Research is a good way of maintaining interest - investigating things that flag up and take me down paths Id never have imaginged which then bring me back to a different place in my writing perhaps.
OVERALL, I find its most important to ENJOY the process. Yes it can be hard slog , is usually slow and longhaul - but it must be something you learn to love .
OMG - how I've gone on.