Rishil has a great post named Manipulating Google Suggest Results - An Alternative Theory.
In short, besides for using raw search volume queries to manipulate the search suggestions and auto-completions given by Google Instant, there may be other ways, such as number of times those matches come up in web page documents.
I think most people who want to go about changing the Google Instant auto-completions will go about it by having "friends" both search for the query they want to show up and also by placing more web pages with those keyword phrases in the documents.
Is it that simple? Well, obviously it depends on how popular the query is. It is very easy to manipulate a Google Suggest auto-completion for [Rishi Lakhani ROFL]. And that is what Rishi Lakhani did using Twitter.
I decided to run a few tests. (lazy ones to be honest). In the first instance, I decided to use the power of twitter and RTs to drive indexed content for my two chosen terms, “ha ha ha” and “rofl” to be appended to my name in suggest.
The test with “ha ha ha” had a strange result, instead of going on suggest, it went on to appear in related searches:
So I pushed ROFL. And as you can see, as of last week
There was a very lively debate about this between Google's Othar, in charge of Google Instant, and Avi Wilinsky during the Google Instant: 6 Months Later panel the other day. You may want to also check that out.
Forum discussion at Sphinn.