I usually rely on the unifying power of alcohol, well it works for us Finns at least. If that is not an option, jokes or a everyday greeting do well.
I'm not an social butterfly myself either, but I change my subject depending on where I am, a first time meeting a local hobby group and I'll ask a bit about how the group funktions and discuss about the hobby. If in a bar alone, I've noticed this to work, "Hey, mind if I join you? Since drinking alone gets a bit dull after a while.".
And the reason why you may ave seemed so stuck up, may not have been solely because you didn't say anything, maybe something in you made them feel like it. People think that I am ~10 years older than I actually am because of how I look and act.
I've actually been told by at least 5 of my close friends that the reason we weren't friends earlier was due to the fact that I seemed extremely upright and hated them because I didn't say anything... Ever. With strangers, I tend to lose the usual warmth I shower over my friends. Thank you for the advice!!!