Going to the bar alone is a great way to meet women. Last night I went to the bar alone and used one opener the entire night, and ended up talking with a lot of hot women and going home with one of them.
What I've found out about approaching women at the bar when you're alone is: women of beauty are almost never alone. This is common knowledge. So you have to approach a pair or group of women usually. Start out with an open-ended question addressed to both women in the pair you approach. Personally, I like the Mystery Method's style of saying "I only have a minute, but..." and giving the women body language like you're about to leave any second while you ask the question. You don't appear threatening that way.
Also, have a brief explanation about why you're at the bar alone or where your friends are when asked. Eventually you'll want to isolate one of the women, which is probably the biggest challenge of approaching a pair of women, but the Venusian Arts Handbook does a good job of explaining this whole process.
Good luck, bro.
Roman