With the jQuery Tokeninput plugin it is easy to add an autocompleting list of entries for a many-to-many association.

#259 Decent Exposure

Every other week you will be treated to a new, free RailsCasts episode featuring tips and tricks with Ruby on Rails, the popular web development framework. These screencasts are short and focus on one technique so you can quickly move on to applying it to your own project. The topics are geared toward the intermediate Rails developer, but beginners and experts will get something out of it as well. A Pro option is also available containing more screencasts each week. This is the full resolution version, a lower reoslution for mobile devices is also available.