I'll start by saying the same thing as many have: try on as many boots as you can in person but if you need to buy online make sure that the seller has an ironclad return policy in case you need to exchange them.
I've seen many boot size threads around here inquiring about how different boot manufacturers size in relation to normal shoe size. Many of us live in areas where we don't have a local shop that has a wide range of boots that we can try on in person and are forced to buy online and guess (to a certain extent) in regards to sizing. I figured that a thread like this would be a good place to compare notes.
I've seen mention of Mondo sizing being accurate but it didn't jive for me when measuring my foot - it always came out small. I found this video explaining Mondo sizing and it says to measure the footbed from your athletic shoe and it was spot on for me:
I'm not completely sold on Mondo sizing - due to the experiences that I've had - but here's a chart from Evo:
Snowboard Boot Size Chart & Mondopoint Conversion | evo
So here are my particulars:
Athletic shoe size:
Nike - 9.5
Adidas - 9.5
Asics - 9.5
Northface - 9.5
27.0 centimeters which equals a size 9.0
Owned snowboard boots/size:
2008/09 Burton SLX - 9.5
2012/2013 ThirtyTwo Focus Boa - 8.5
2013/2014 Flow Talon Focus - 9.0
Just got the Flows and I'm really impressed with the fit, how light they are, and their minimal profile. I didn't have a chance to ride them before the season ended but I'm pretty sure that they're going to be awesome.