My guy didn't want to just do letters either. So we had him spell out things that were important: his name, "mom", "dad", etc.
We did do shapes first (not saying there's a scientific method there, just that's what we did): drawing, coloring, etc are usually more fun and you can make "pictures" with shapes. We'd work on a circle for a week and we'd take walks around the block having a "contest" who could find the most "circles" on the walk (tires, dream catcher on someone's porch, etc). Then we'd draw things we saw that had that shape (car, because the tires are circles), that kind of stuff. That helped develop his ability and interest in sitting down and drawing or writing.
We practiced the letters and their order by singing, and an alphabet train puzzle that took up then length of the den, The Letter Factory did good at showing that the letters have shapes, sounds, and meaning (and he just liked watching the thing anyway). We did do a lot of things with drawing and "copying" things. I'd draw a big circle, and he'd draw the same. I drew a little circle and he'd copy. (We'd take turns and I'd copy what he was doing too). The puzzle books (dots, tracing, mazes, circling things, etc), and the pages where you draw lines from one thing to another, all that are kind of fun. We did play with a lot of play doh and games where we worked on coordination. We'd use different mediums when practicing writing: finger paints, water colors, "real" paints, mosaic chips (left over from other projects) that we'd put in the garden, sidewalk chalk, pencils, map pencils, crayons, ink pens, and markers. He learned words he "liked" first (as well as lots of drawing and playing around too). We'd see a cool car and on butcher paper draw, color, or paint some cars, and I'd show him how to "write" car. I'd draw it and make the "c" for "c-c-c-car!" and then he'd copy. He would write car a few hundred times over the course of a week, and have little car pictures (little more than squiggles with 2 circles for wheels). Stuff like that. We also got on Kokakgallery.com and made a photo book called "The ABC's of Daddy" for father's day that year that he was learning all that (age 3): stuff like A is for Adore (photo of the boys and their dad gazing at each other), love that is Absolute (a wedding photo), travels Abroad (photo of London), and Aquarium (photo of them staring at some sharks at the city aquarium)....etc, etc. Obviously not words he could read, but he understood the concept that letters make special sounds and are important. That helped him. Now he's 4 1/2 and does know how to write, and he thinks that's pretty cool. It does take time though. I don't think any child sits down and says "Let's learn to write today" and just sit there for long. (I'd suggest 10 minute blocks because for many little boys-mine included-10 minutes is a long time).
LOVE Sam I Am's rainbow letter idea. That's cool! I'm going to start doing that. :)