Open Brainstorms

Create user profiles while booking the ticket through internet .(assuming all tickets are booked electronically here) It helps in obtaining statistical data and all profiles can be updated with points as they book or removed as they cancel.

Automatically give the frequent flyer points to everybody, based on the creditcard or ID on check in. No need to manually "sign up" for it...

Also, airlines should place advertising on the backs of boarding passes and simplify the packaging. Right now, the pocket that boarding passes are placed in has hundreds of words in small fonts that no one reads. Why not replace all that nonsense with one single large-font / graphical promotion? By way of comparison, look at Google's home page vs. Yahoo's. Better to have one thing noticed by a lot of people than a lot of things noticed by no one!

Using electronic kiosks, after check-in but before printing boarding passes, add a screen that promotes the FF program. It could either give a new FF# (as suggested by Anand in comment #2), or suggest uses of FF points/miles that could be instantly redeemed. This would provide a fast way to promote the FF program without taking much additional time

Queing at chen-in is long enough as it is - it should not be made longer, or more complex by adding the FF program sell to it. If you aren't a frequent flyer by the time you've booked your ticket, you won't be by the time you check in. Give them a form (back of Boarding Pass?) with details on how to SMS your application while you're waiting for the plane to board/take-off - and double the points for the first flight if they do. Think about the customer - not about the company.

Hey! I just noticed, are you suggesting that the airline SELLS its frequent flyer program to its customers?!! Well, THAT will be a hard sell. Loyalty programs are designed to REWARD the customer when they spend more and/or more consistently with the vendor. You want to CHARGE the customer for the right to benefit from giving you their business?!!! I wouldn't surprised to see the attemtpt (especially in America) but any measure of success would be sad.

Give everyone a frequent flyer number by default, a person should not need to sign up.

Through the reservation system, the airline can already get all the data needed to sign someone up for their points program - simply ASK at checkin, if the person says okay - give them a temporary card and number (full kit arrives in the mail) some bonus miles and the current flight miles. It ain't rocket science.