I spent a lot of time helping my students work on their artist's statements last semester, so I'll share my methods with you here. It doesn't have to be a painful experience.
Artists are required to write statements for a number of occasions, like grant, residency, or fellowship applications to exhibitions or proposals. You'll need to tailor your statement for the intended audience and will probably need to revise it frequently.
The statement is your chance to get the audience interested in your work, not unlike a sales pitch. Be honest and brief. A well written statement will help your audience see something in your work that they may not have noticed on their own.
Outline your statement as follows:
1st paragraph: Thesis statement in 1-4 sentences. This is a quick introduction. Include your name, the theme of your work, and the medium you work in.
2nd paragraph: Help us understand your creative decisions. Describe your personal connection to the subject matter or composition. Explain why the medium you've chosen is most appropriate for your work.
3rd paragraph: End with a very brief bio, with 2-3 highlights of your career, and current or near future projects.
Consider your audience. For example, a statement written by a printmaker and addressed to a group of professional lithographers would be different than one written for the general public.
Below you will find some writing exercises designed to help you describe and explain your work. You'll use your answers to create your rough draft.
Start by making lists. The lists are for your eyes only, so just write down everything that comes to mind without editing along the way. Not everything will end up in your final draft, but the more thoughts you get down on paper, the more you'll have to pull from as you draft your statement.
Write about one series or group of similar works at a time. How does this compare to your work as a whole? We often need to revise our statements as our work evolves, or as we evolve.
LIST #1: free association: Write down everything that comes to mind when you look at your art. Remember to consider groups of similar work if you work in different media or address difference subject matter. Then consider the relationship between these groups of work.
For example, I work in a variety of media and in series based on different ideas. The overarching theme is our relationship to nature and finding hope and wonder in a rapidly changing environment.
My list: scratchboard, illustrations, fairytale, folklore, old prints, classic illustrations, children's books, narrative drawing, light and dark, chiaroscuro, story-telling, nature, animals, animals and people, abandoned spaces, environmental, fear, anxiety, woods, trees, subsistence, survival, communicating with animals, campsite, abandoned spaces, resources, foraging. 
LIST #2: Sources of inspiration: There's no correct answer. Be honest! As you begin to write, you'll discover which of your experiences are showing up in your work.
Do they come from choosing between countless variations on cereal at the Piggly Wiggly? Chopping down trees in secrecy? Reading the funny pages? Using leftover paint? Looking at other art?
My list: walking in the woods, travel in Brazil, seeing bears every day while living in Virginia, birdwatching, touring with my band and seeing how other people pursue their dreams, studying natural history illustration, printmaking experience, picture books, illustrations, travel, working on farms, environmental fear and anxiety, traveling in Turkey, considering mortality, abandoned spaces, memory, imagination, reading George Saunders, Kurt Vonnegut, Cormac McCarthy, folklore.
LIST #3: Materials/Media: What materials do you use, and why? What makes them appealing to you or appropriate to your work? IF you are working in an unfamiliar medium, briefly describe the process. It's not necessary to describe familiar media, like paints, pastels, or pencils.
Why a sculpture and not a print? Why soap and dryer lint and not pastels? Sell us your medium. Make us see why one medium works better for you than another. What qualities does it have?
A common mistake my students make is to say they use a certain tool because it's "easy." Well, what makes it easy and why is that important? "Because it's easy" implies that you are lazy and don't want to exert too much effort, but that is presumably not what you mean. Maybe you like a tool that helps you work quickly and spontaneously to record impulses or fleeting moments. That's a lot more interesting to your audience, right?
My example:
scratchboard - looks like etching, looks like old illustration, great for delicate lines, great for details, comes as black field and I scratch into it to reveal light, pull light out of dark, great for creating light, high contrast in value, dramatic, each mark is very deliberate because it's hard to correct, meditative
LIST #4: artistic influences: Most of us want to be original and don't like to be compared to other artists, but it's extremely rare to create something entirely new. Understanding and identifying the similarities between your work and others provides context for your audience.
Be general and specific at first and sort it out later. You could include a broad concept, like painting from observation, or a movement, like Impressionism, or a specific artist, like Jasper Johns.
Write down everything that comes to mind. The connections between your work and your influences might not be obvious right away, but you'll figure out the specifics of that connection in the next stage.
My list: Redon, Gustave Dore, Edward Gorey, Symbolists, classic children's book illustration, Rembrandt, chiaroscuro, engraving, natural history illustration, children's books, Old Masters, painting from observation, drawing from observation, naturalism
For example, I am influenced by Rembrandt's use of dramatic lighting. I wouldn't want to compare myself to him in a general way and look like a third rate artist, but I might say that I use high contrast in light and dark as developed by the chiaroscuro painters of the Renaissance.
LIST #5: Specifics of style and medium. Consider the following aspects of your work and write down everything that comes to mind. Describe the color choices or combinations and the methods or techniques used. Tell us how the medium is used, as your audience may not be experienced.
The most important question to ask yourself as you study your own work is WHY? Why red and orange instead of blue and violet? Why soft instead of hard edges? Why abstraction instead of photo-realism? Why small and not large? Are there transparent layers or areas of opaque color? Are the edges hard or soft? Are the shapes well defined or merely suggested?
For example, (I'll address my most recent series, done on scratchboard)
Scratchboard is masonite coated in a fine white clay and coated in black ink. The ink is removed with a sharp tool to reveal the white beneath. The subtractive process requires lights to be pulled from the dark and is perfectly suited to my interest in creating atmosphere through dramatic lighting. It holds delicate lines perfectly, as they are etched into the surface with a sharp tool, such as an exacto blade or needle. The medium evokes old etchings, picture books, and 16th-18th century scientific illustrations.
3) Read lots of other artist's statements and identify what makes them interesting, or boring, or even infuriating. Hopefully they reveal something that makes their work a little more interesting or easy to connect with.