If customers can't choose it, don't show it
Another way to make life easier for your customers is to eliminate unavailable options (a.k.a. prevalidation). If I can't choose an entry, don't show it to me in the first place.
Then Don't Offer 2003 http://www.google.com
I'm choosing the date to stop running an ad on Google. 2003 is listed as one of the year options. (In fact, it was the default selection.) When I choose a date in 2003, however, the site informs me that 2002 is the latest year I can select.
It's frustrating to select a default option and then find out it's invalid. If I can't go past 2002, Google shouldn't even present 2003 as an option.
Why Not Tell Me It's Sold Out? http://www.ticketmaster.com
I tried to buy tickets to a sold-out concert at Ticketmaster. Unfortunately, the site never actually told me the show was sold out. Instead, it displayed the show's information and let me select tickets before it actually spilled the beans that tickets for this event were not available.
Even then, this ambiguous error message still leaves me scratching my head. If Ticketmaster knows the event is sold out, then why not just say so? It's no wonder customers wind up ticked off at the ticket behemoth.
From the customer...
"Ticketmaster.com is a case study in how not to create an online service... Why can't it just tell me there are no tickets available for a particular event? Instead it keeps prompting you to try different combinations of quantity and location because you figure if it doesn't indicate that it's sold out, there must be at least one little ticket waiting to be found."
Only Show Available Times http://www.peapod.com
Peapod normally offers a range of delivery times throughout the day. On Saturday, however, the grocery service only has a limited number of slots available. Instead of displaying Saturday's later time slots that can't actually be chosen, the site shows only the times that are valid selections.
This is helpful because I don't want to select a time slot and then find out it's unavailable. By preventing this sort of wrong turn, Peapod prevents backtracking and gets me one step closer to checkout.
Easy Picking http://www.easyjet.com
Here, EasyJet eliminates invalid airport options. Selecting Athens as my "From" point automatically filters the options in the "To" field to display just "London Luton," the one destination available from Athens. After all, it's easyJet that knows exactly where their planes fly, not their customers.