[notatoad]

it's not so much that users won't read, it's that they read as little as they think they can get away with to make a decision. the best way to solve this problem is to take the decisions away from the user. instead of asking if they want to sign in, give them a username/password form. most of your users will probably be able to fill out the form. no decisions, no reading necessary, no problems.

the users that aren't able to fill out the form they will be unable to continue until they read. they will start looking for the "sign up" button. make it big and obvious for them. still, nobody has made any decisions. the user takes the only path available to them.