Before we even begin trying to form letters, we spend a huge amount of time in what some would call "pre-writing". Mainly, this has been nearly accidental in our home and more a part of playtime, although I now see how beneficial it has been. It's simply scribbling at first, gradually developing into drawing and doodling. I keep a drawer accessible for the "older" children with paper, various pencils, erasers, Ferbies, and such for them to use whenever they want (except right before a meal - we need to set the table!). Magna-Doodles have been invaluable for this purpose, too, especially for toddlers that don't necessarily get free Ferby access! :) We have large Magna-Doodles in several rooms of the house and one or two small ones in the van.
I gently encourage making circles and tracing simple lines as a game of sorts fairly early on (about 2 and 1/2). As the children get older, I keep on the lookout for attempts at letter formation. I do not want them forming letters incorrectly - even for playtime. An attempt to make an "H" turns into an impromptu two minute lesson in handwriting :) I really think this kind of daily "play" has been instrumental in preparing the children to write... it hasn't hurt at least. Beginning at around age 4, we have also used Ready Writer and have enjoyed it very much. Although, I'm not sure it has been any more beneficial than the stylus practice we've had through doodling and drawing. Shug, in particular, seems to be beyond it already and it hasn't been as helpful for her. Others have recommended inexpensive activity books with mazes for pre-writing. I haven't found these to be as profitable as good old-fashioned scribbling and tracing, though.
With a little encouragement and some materials, doodling and pre-writing can be a natural part of the day - one the children may even ask you for! :)