The simplest approach is in asking: what is good for us here and now? Should we limit immigration to as few people as possible, or should we allow to come as many people as possible? Which of the two options will be more beneficial for our economy, or in other words, which of this two options will make most of us wealthier?
I did this analysis, and arrived with the Freedom of Migration Act proposal, http://www.freedomofmigration.com .
For people who want debate this issue further, I opened a special forum: www.immigrationdebate.net