This might sound mean, but when I'm playing online, and can see the exact dollar figure that he has left, I subtract $1.25 from whatever he has left, and bet that.
I.e. if he has $47.63, I bet $46.38.
If his response is an immediate "wtf?" or even a lengthy delay, I let him know that the $1.25 is for bus fare. It amuses me, especially in a tournament where the chips are meaningless.
In a live game, I usually just bet about 2/3 of their chips so they know I'm priced in, even if they suspect a semi-bluff and contemplate the re-raise.