“I love working here, and because of that I need to let you know that other opportunities are becoming financially better alternatives. Would you reconsider the maximum salary that you think I’m worth here, so that we can keep working together?”
I think this is the best answer. It shows you _want_ to work there still which your boss will hopefully appreciate. Pretending to already have an offer could come back in your face.
This is going to be more or less my strategy when I ask (rather soon). I do love working where I am. The only problem is that other offers are starting to come in, and I know that they'll be offering a lot more money. :(
