Recently my wife and I were trying to work out why she couldn't submit her address details to a website, even though I could. As we watched her behaviour in filling out the form, we encountered error after error: or rather, exceptional circumstance after exceptional circumstance. And it was clear that very few of the circumstances had been considered, that error handling was the absolute bare minimum, that the form was set up to be almost a trial to use. The postcode lookup part of the process was probably the source of the most unhandled exceptions: difficult if not impossible for the power user to flow through; unwieldy for the standard user; of almost no benefit at all to the web newbie.
People still think of their workflows on the web like they're workflows. Over here there's the start; over there is the goal; somewhere in between there might be some intermediate stages, but ultimately you go from over here to more or less over there eventually. It comes as something as a shock to most people that their beautiful webform does not encompass a workflow: the web has holes all over it; the user is a ball bearing and your application is a pinboard:
Above we see a slightly clumsy metaphor for your web application. The end point of your own particular "workflow" isn't even something visually obvious like the shark's head. It's, oh, let's say that small metallic gate in the bottom right with the red doors (luckily for you) open. The best you can hope for is that the user caroms through your website hitting as few pins as they have to and ends up in one of a trillion end points, that they don't close the browser, or reach a form error, or silently lose their submission, or navigate elsewhere in irritation.
In fact, it dawned on my wife first, and then gradually on me, that postcode lookups are not intended to directly benefit the user filling them in. Instead, they're meant to force the user---remember that phrase---to provide a canonical address, and not the address. That is, the user comes to your site with an opinion about where they live and limited good will about your "product", and the postcode lookup is a mechanism for forcing them to discard the former, while the application as a whole is trying hard to get them to keep hold of the former.
Good luck with that.
Richard Rutter figured out the dirty secret behind postcode lookups---that they're not for the user---long before my wife and I. In order to mitigate this natural tension between forcing the user and keeping them happy, he's done a sizeable chunk of work to condense the postcode lookup pattern here. Along with a quite lively and informed conversation in the comments, this post nails much of the core of the pattern that lookup needs. Much of what's frankly miserable about using a postcode lookup is indeed tackled there, but there's an important omissions that I think needs dealing with. Roughly speaking, that is: never force your users to pretend to be someone they're not.
As an example, consider Spotify. Since inheriting a slightly ropey eMac, I've been able to listen to Spotify, and I like it. I think Spotify is a gamechanger in the field of streaming music. I've heard albums on Spotify that I would never have bought. And yet: I would never consider purchasing Spotify Premium. The obvious barrier for me is that I use Linux, and there are no native Linux binaries for the Spotify desktop client.
People keep telling me that Spotify runs really well on the Windows emulator WINE. I'm sure it does. But that misses a more fundamental point: if something wants me to enter into a relationship with it, commercial or otherwise, I should not have to pretend to be a demographic I'm not in order that the relationship can be properly fulfilling. I'm not a Windows user, and it's an affront to a paying customer to expect them to make out that they're a type of user that they're not if they want to buy your stuff. More consisely: I don't take offence at the interface requirements; I take offence at what they imply about the respect for my needs as a user.
With that in mind, consider the first decision block in Richard's workflow:
does the user know their postcode?
As this is the first pin the user's pinball hits, then this is the one that alters their final resting place the most. This is the critical pin. And what does it tell me, a user who knows his postcode but also knows his address, and doesn't want to bother with lookup? It tells me that I have to pretend to not know my postcode if I want to be in a situation where I don't have to put it in. I have to play games with the application, and mask my true intentions. No, the postcode lookup system must allow for users who simply do not want to fill out their postcode. Postcode lookups should therefore begin with a simple choice of two buttons by a traditional form label. The buttons should not make any assumption about the user's reasons for choosing either route:
Address: [LOOK UP POSTCODE]
[JUST LET ME TYPE MY ADDRESS]
When you press the first button, both buttons should still remain on the page: the user might decide they wanted to press the second one after all. In fact, as we're probably using AJAX here, the minimum necessary modification to the form is just to add a postcode box (and to move and maybe change that button) like this:
Address: [ postcode ] [LOOK IT UP]
[JUST LET ME TYPE MY ADDRESS]
You should still present the user with the ability to change their address. The "speed bump" of having to press the button is what works in your favour, is what gets you access to canonical data; anything more than a bump will run the risk of walloping the user right off your pinboard.
If at any stage the user clicks on the second button, the webform should then change to this view:
Address: [ Address, either as a set of textfields or as a textarea ]
[ postcode ] [LOOK IT UP INSTEAD]
For simplicity, I'm almost tempted to drop lookup button altogether at this point: the user has made their choice, after all. But you should never make it difficult for users to go back in a workflow, especially when (almost certainly) the browser back button will have been disabled by these shenanigans.
In comment 10 on his blogpost, Richard agrees with the idea of one big textarea for the address: possibly even including the postcode in that textarea! Again, the simplicity is appealing. You could do all sorts with this to make the user's experience easier: regex matching behind the scenes would retrieve the postcode; the address could even be automatically split into lines rather than setting real estate aside for the user to split them up themselves. It sounds great, but it's not an established pattern, and I think a lot of users---especially power users---would mistrust it. Better to go with Address 1, Address 2 etc. even though from a data perspective they're a horror, slightly improved---but, again, made more complex for the user---by labelling the last address line as "town". But this last detail is up to you. Do some A/B testing. See how it goes.
Richard's workflow, with the addition of the basic prototypes above, permits us to move towards as usable a system as postcode lookup will ever be. Usability means the least number of pins on your pinboard, and exactly the right pins, the ones that nudge and tilt the user just enough. And so we end up with a system that still satisfies your original remit---to nudge the user towards using your shiny, expensive, time-consuming, postcode lookup service, with all its concomitant costs in development and maintenance---while catering for the users who simply do not want to, who will never want to, and who will actively object to your site giving them short shrift if they try not to use it.
I'll make a prediction here, that the users who try not to submit a postcode will tend to be the users you want: the digital natives, the users in flow, the people who will buy ten things from a polished user interface without even stopping to think about it. When they reach that very first pin on the board, they briefly want to be your application's friends on the web. Your application should consider itself honoured. The least it could do in return is be polite.