Are you determined to go to a particular store? Apple? One of the networks? Carphone Warehouse? If you're only willing to buy from one network (e.g. Orange) then that limits your choices
How many stores are there wherever you're planning to queue? The more places there are, the more places there will be for other people to queue. Some of them will be more upfront about how much stock they're expecting to get. If there are multiple stores of the same brand nearby, often only one of them will have the new iPhone.
Does it matter how long it takes for you to actually get the phone? If you have to be at work at 09:00 and the store opens at 08:00, then you'll want to be near the front, even if the store has plenty of stock to go around
When I queued for the iPhone 4 at the O2 Store in Bristol, I was not the first in the queue at about 23:30 the day before!