For the first nine months I was at my dojo, we did a month-long (seven classes) Intro course, which people could also repeat. I did it a couple of times before I was comfortable joining any other classes, because my forward rolls were the despair of the dojo.
This fall we switched to a one-week (two classes) Intro course. The idea was to integrate people into the regular classes quicker, and to have less obstacles in the way of people joining (previously there was some sense that they should wait till next month, if they came mid-month). The classes on Monday and Friday immediately after Intro were Basic, taught more gently than the regular, but with a mix of levels.
It's hard to say which was more successful, because in the spring we were listed with the Experimental College and in the fall we weren't, and that had a huge impact on how many new people came at all. I thought, on the whole, that the new people found the shorter class more intimidating. It wasn't enough time to teach much rolling, and when Basic started with rolls the new people seemed to feel at a loss.
One of the most successful beginner things we did, I thought, was to introduce some ki tests and "cute ki tricks" at the beginning. The "aha" of being able to do something surprising seemed to come quicker and easier with unbendable arm or unliftable stance than it did with any of the throws. And I think "aha" is important in bringing beginners back--it surely was for me.
The other practical thing I can say is that encouraging a few more experienced students to come to the Intro classes helped a lot more than it hurt. It was important not to treat the classes as a chance for the experienced people to train together (scary to watch!) but having sempai as ukes for sensei, and as partners for the newbies, worked a lot better than having sensei try to do it all him/herself. We have one high-ranked student who diligently comes to almost all of the Intro and Basic classes, and he had a lot to do with my sticking with aikido through all those rotten rolls. (And the expression on his face when he threw me into my first full-speed forward roll made it all worthwhile.)